"Storing.—The cleansing being finished, the future disposal of the porter determines the brewer what course to pursue,—either to remove the brewing into cellar-stock, in the same casks into which it was tunned, to be afterwards racked into barrels, to be sent out to customers ; or to store it in a vat for the same purpose.
In the first instance, supposing the porter to have been made in the spring, to be ready for delivery during the summer, such old ale in the brewery as may be fit for the purpose is put down with hops, 2 lbs. to each barrel, and kept until required. In racking the porter from the pipes into barrels, 9 gallons of the old ale are first delivered into each barrel of 36 gallons, and smaller casks in proportion, and the barrels filled up with porter. A table-spoonful of heading is added; this consists of equal parts of alum and carbonate of potash; also the same quantity of orange powder; last, about the third of a pint of finings made up with vinegar and isinglass. Some brewers keep a supply of London porter by them, and add a gallon or two to each barrel racked; and it may be easily imagined, that the provincial production will be all the better for such an addition. In porter-brewing one thing is essentially requisite, the brewer should contrive to make a strong-bodied, fine-flavoured, malt liquor, which, if it does not pass for London porter, will pass on its own merits, which is all that is required.
When porter is required to be stored, a quantity equal to three-fourths of the measure of the vat must be overturned into it fresh from the stillions, immediately after having undergone the process of cleansing. One-fourth of prepared old ale is added to fill up. The manhole is battened down air-tight, sufficient space being left for the rise and fall of the liquor within, a slack-spile or vent-plug being inserted at the same time on the head of the vat.
A trial stopcock is placed about a third part up, by which the brewer has access to judge of the progress the contents are making towards ripeness. In two or three months it is ready, when the same additions are made in racking as previously described in racking from the pipes.
It is at this stage that brewers differ as to the additions to be used to obtain the flavour of London porter. Decoction of bark, and porter extract. are sometimes used ; but, as has been already mentioned, if the malt liquor is really good, there is not much occasion for using such ingredients.
The reader must take the preceding description of brewing malt liquor, not as the general method adopted by provincial brewers of making porter, but rather as affording general information of the mode of conducting the process, and preparing the foreign ingredients, by a judicious management of which, an imitation of London porter can be produced."
"Brewing and Distillation" by Thomas Thomson and William Stewart, 1849, pages 286-287.
I'm accustomed to the blending Porter after vatting, but this something different. Here already aged Ale is being added at the start of the vatting process. My guess - and remember this is just a guess - as to the reason why they did this is that it speeded up the ageing process. The old Ale would presumably already have the desired aged flavour and the purpose of vatting would be to meld these with the Porter, rather than to leave the Porter to acquire aged character itself.
Note the proportions of old Ale to fresh Porter: 1 to 3. The Amsdell Porter had 65 barrels of old Ale to 205 barrels of freshly-brewed Porter. Which is just about 1 to 3. The big difference, however, is that the old Ale was added at the start of primary fermentation rather than at vatting time.
Labatt in Canada were doing something similar with their Brown Stout in the 1890's, adding about 10% old Ale. Though it's not clear from their records at what point it was added to the fresh Stout.
I'm pretty sure that what's meant is old Ale rather than Old Ale. That is, Ale that had been hanging around the brewery for a while rather than Ale that had been deliberately brewed to be aged. As this passage from page 277 of the same book makes clear:
"it is very useful to make two or three brewings of porter to take up hard ale, which answers excellently, when it has been properly treated with hops"
"Hard Ale" being Ale which had got too old.