October Beer has a long history, already having been around long before 1700. It's believed to be the direct ancestor of Stock Ales and Barley Wine. Why was it called October Beer? Because it was brewed in October, silly. The best months for brewing were March or October because the ambient temperature was optimal. Though some thought March best, most brewers preferred October because it was followed by 6 months of relatively cool weather, perfect for maturing beer.
"A particular way of Brewing strong October Beer
There was a Man in this Country that brewed for a Gentleman constantly after a Very precise Method, and that was, as soon as he had put over all his first Copper of water and mash'd it some time, he would directly let the Cock run a small stream and presently put some fresh Malt on the former, and mash on the while the Cock was spending, which he would put again over the Malt, as often as his Pail or Hand-bowl was full, and this for an Hour or two together; then he would let it run off intirely, and put it over at once, to run off again as small as a Straw. This was for his October Beer: Then he would put scalding water over the Goods at once, but not mash, and Cap them with more fresh Malt that stood an Hour undisturbed before he would draw it off for Ale; the rest was hot water put over the Goods and mash'd at twice for small Beer: And it was observed that his October Beer was the most famous in the Country, but his Grains good for little, for that he had by this method wash'd out all or most of their goodness; this Man was a long while in Brewing, and once his Beer did not work in the Barrel for a Month in a very hard Frost, yet when the weather broke it recovered and fermented well, and afterwards proved very good Drink, but he seldom work'd, his Beer less than a Week in the Vat, and was never tapp'd under three Years."
"London and Country Brewer" (1736).
Here's a rather more detailed description.
"A Philosophical Account for Brewing strong October Beer. By an Ingenious Hand.
In Brewing, your Malt ought to be sound and good, and after its making to lye two or more Months in the Heap, to come to such a temper, that the Kernel may readily melt in the washing.
The well dressing your Malt, ought to be one chief Care; for unless it be freed from the Tails and Dust, your Drink will not be fine and mellow as when it is clean dressed.
The grinding also must be considered according to the high or low drying of the Malt; for if high dryed, then a gross grinding is best, otherwise a smaller may be done; for the Care in grinding consists herein, lest too much of the Husk being ground small should mix with the Liquor, which makes a gross Feces, and consequently your Drink will have too fierce a Fermentation, and by that means make it Acid, or that we call Stale.
When your Malt is ground, let it stand in Sacks twenty-four Hours at least, to the end that the Heat in grinding may be allayed, and 'tis conceived by its so standing that the Kernel will dissolve the better.
The measure and quantity we allow of Hops and Malt, is five Quarter of Malt to three Hogsheads of Beer, and eighteen Pounds of Hops at least to that Quantity of Malt, and if Malt be pale dryed, then add three or four Pounds of Hops more.
The Choice of Liquor for Brewing is of considerable advantage in making good Drink, the softest and cleanest water is to be prererr'd, your harsh water is not to be made use of.
You are to boil your first Liquor, adding a Handful or two of Hops to it, then before you strike it over to your Goods or Malt, cool in as much Liquor, as will bring it to a temper not to scald the Malt, for it is a fault not to take the Liquor as high as possible but not to scald. The next Liquors do the same.
And indeed all your Liquors ought to be taken as high as may be, that is not to scald.
When you let your Wort from your Malt into the Underback, put to it a Handful or two of Hops, 'twill preserve it from that accident which Brewers call Blinking or Foxing.
In boiling your Worts, the first Wort boil high or quick; for the quicker the first Wort is boiled, the better it is.
The second boil more than the first, and the third or last more than the second.
In cooling lay your Worts thin, and let each be well cooled, and Care must be taken in letting them down into the Tun, that you do it leisurely, to the end that as little of the Feces or Sediment which causes the Fermentation to be fierce or mild, for Note, there is in all fermented Liquors, Salt and Sulphur, and to keep these two Bodies in a due Proportion, that the Salt does not exalt itself above the Sulphur, consists a great part of the Art in Brewing.
When your Wort is first let into your Tun, put but a little Yeast to it, and let it work by degrees quietly, and if you find it works but moderate, whip in the Yeast two or three times or more, till you find your Drink well fermented, for without a full opening of the Body by fermentation, it will not be perfect fine, nor will it drink clean and light.
When you cleanse, do it by a Cock from your Tun, placed six Inches from the Bottom, to the end that most of the Sediment may be left behind, which may be thrown on your Malt to mend your Small Beer.
When your Drink is Tunn'd, fill your Vessel full, let it work at the Bung-hole, and have a reserve in a small Cask to fill it up, and don't put any of the Drink which will be under the Yeast after it is work'd over into your Vessels, but put it by itself in another Cask, for it will not be so good as your other in the Cask.
This done, you must wait for the finishing of the fermentation, then stop it close, and let it stand till the Spring, for Brewing ought to be done in the Month of October, that it may have time to settle and digest all the Winter Season.
In the Spring you must unstop your Vent-hole and thereby see whether your Drink doth ferment or not, for as soon as the warm Weather comes, your Drink will have another fermentation, which when it is over, let it be again well stopped and stand till September or longer, and then Peg it; and if you find it pretty fine, the Hop well rotted and of a good pleasant taste for drinking.
Then and not before draw out a Gallon of it, put to it two Ounces of Ising-glass cut small and well beaten to melt, stirring it often and whip it with a Wisk till the Ising-glass be melted, then strain it and put it into your Vessel, stirring it well together, stop the Bung slightly, for this will cause a new and small fermentation, when that is over stop it close, leaving only a Vent-hole a little stopp'd, let it stand, and in ten Days or a little more, it will be transparently fine, and you may drink of it out of the Vessel till two parts in three be drawn, then Bottle the rest, which will in a little time come to drink very well. If your Drink in September be well condition'd for taste, but not fine, and you desire to drink it presently, rack it before you put your Ising-glass to it, and then it will fine the better and drink the cleaner.
To make Drink fine quickly, I have been told that by separating the Liquor from the Feces, when the Wort is let out of the Tun into the Underback, which may be done in this manner, when you let your Wort into your Underback out of your Tun, catch the Wort in some Tub so long, and so often as you find it run foul, put that so catched on the Malt again, and do so till the Wort run clear into the Underback. This is to me a very good way (where it may be done) for 'tis the Feces which causes the fierce and violent fermentation, and to hinder that in some measure is the way to have fine Drink: Note that the finer you make your Wort, the sooner your Drink will be fine, for I have heard that some Curious in Brewing have caused Flannels to be so placed, that all the Wort may run thro' one or more of them into the Tun before working, by which means the Drink was made very fine and well tasted."
"London and Country Brewer" (1736).
Such beers were incredibly strong, with starting gravities over 1100 and an ABV of somewhere around 10%. They were more often brewed in private households than in commercial breweries. Which, given the length of time they took to make, I suppose isn't surprising.