Similarly, I'm not sure if the original oatmeal stouts actually had oatmeal in them.
Yes they did - they were the subject of a bit of a patent fight in the 1890s between Rose & Wilson of Yorkshire who seem to have been the first to patent the idea, and Maclays of Alloa who sort-of lost the legal battle but ended up being the ones to make a success of oatmeal stout - see eg
here.
They then got a bit of a bad name because London brewers between the wars felt they had to have an oatmeal stout in their lineup, so they would put 0.5% oats in their regular stout and market the same beer as stout and as oatmeal stout.
There seems to be a bit of a divide, British brewers, at least the traditional ones, sell oyster stout to be
drunk with oysters (eg
the Marston one) whereas US brewers make oyster stout
brewed with oysters in the brew. British brewers have started doing it a bit now, but the original oyster stouts did not contain oysters.
Many brewers use eisenglas/isinglass for fining - it's a form of collagen from fish swim bladders, typically part of the waste products from cod these days. IIRC Guinness gave it up last year, so they could claim to be vegan.
Boak and Bailey have access to quite a lot of primary sources from Guinness, see eg
here from 1943 - Old Beer Storage (old, sour beer) was just one of the components of Guinness at that time.