My mum, brought up in Lancashire, remembers jam always being "Mixed Fruit" and always rationed, so the concept of "off ration" implies to her some possible arrangement with the WI, who, I believe had extra sugar made available to deal with available fruit. The recent TV series "Wartime Farm" showed such an operation. Taunton is of course in a fruit growing area.
I understand that plums made up quite a large proportion of "mixed fruit", as they would be a big proportion by weight of the home grown fruit crops.
It is difficult to remove the stones from raw damsons. My mum never even bothered fishing them out of the jam pan after cooking. Does your uncle remember stones in the jar?
There is also a much lighter coloured type of plum, the greengage, which also makes a delicious jam, but is rarely seen in commercial quantities.