There are a few different types of brake accumulators for the 108.
There is no dimensional difference between the AN 6227 and MS 28775 series O rings. There was a movement years ago to replace the old AN (Army-Navy) specs with MS (Military Specification) part numbers. In fact, you can see this in something like an AN 470 rivet. They became MS 20470 rivets. Same rivet, new number with a more standardized number (that has to be five digits). Both O rings are made from the same material even, Buna N. An AN 6227-7 or MS 28775-012 O ring should measure at .070″ in width and .364″ ID. If you’d like to read all about it and more, check out Air Force TO 00-25-223. Just google it.