It's been an interesting and productive week. Mostly consumed in a discussion of 19th century beer codes. It's been useful to get some other views. Many thanks to those that have provided them.
Particular thanks to Gary Gillman who pointed me in the direction of a Fuller's ad from 1893. Why am I so grateful for that? Because I have photos of Fuller's brewing records for 1887. The two match up nicely. So I can see the exact details of the beer, how they were described to the outside world and their price. I'm so excited.
Unfortunately, there are only a couple of K's in there. XK, AK, XXK. But I can detect a pattern in their use. X represents the standard base strength of 1055º. It's worth mentioning at this point that 1055º was a highly significant gravity at the time. Tax was levied per "standard barrel", 36 gallons at 1055º. You can find a fuller explanation of a standard barrel here http://www.europeanbeerguide.net/ukstats.htm#bulk
It seems to me as if A is being used to designate a beer below the standard 1055º gravity. The K is added to beers which are not mild, i.e., those not sold young: Bitter, Light Bitter, Old Ale.
I'd forgotten to mention that there is a use of A that I do understand. During WW I, breweries introduced a new low-gravity Mild that had its gravity and price regulated by government. They had designations such as GA (Government Ale) and Ale 4d (4d was the price per pint). These beers survived into the interwar period and were the cheapest beers sold in pubs, selling for 4d a pint when standard Mild cost 5d or 6d and Bitter 7d or 8d. Barclay Perkins brewed three Milds in the 1930's: A, X and XX at gravities of around 1032, 1037 and 1042 and a price per pint of 4d, 5d and 6d.
The A in Fuller's AK seems to fit that convention. Of course, using this method, the names XAK and XXAK would make no sense. But I gave up expecting a universal explanation years ago. A partial one will do fine.
Before anyone mentions it, I had noticed that the IPA is stronger than the standard XK Bitter. It's also the most expensive beer, in terms of price per gravity point, costing 50% more than Porter. With the weakest beer having a gravity of 1050, there wasn't anything very sessionable in Fuller's range.
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