I've never been able to find many documents about the K hubs beyond how to repair them and a few exploded diagrams - actually, they look quite scary and I hope I never have to completely dismantle one for any reason... well, actually, that would be easy, it would be re-assembling it that would bother me.
Yes, it is entirely possible that the wheel was replaced, though it does seem to match the bike - all parts painted black in the same enamel as the frame, though everything had a coating of matt lead paint on it (Presumably from the war). I'm happy with 1936 anyway, it would add up with some other parts on there.
Oh, and I see you have indeed worked with cotter pins, I hate those things, that two hammer trick may come in useful if I ever have to take the pins out (such as if I ever obtain an oil bath that fits) as I usually end up injured.
All this ounces stuff sounds light to me... I'm usually not fussed on weight and prefer heavier bikes, but then I am quite tall and heavy so it stands to reason that I would find weightier bikes easier to control.
Anyhow, I've exceded the 2-post limit, and so can now add those thumbnails here, though I'm happy with 1936 as we both guessed at that and as such it is likely right, unless the wheel was indeed replaced.
Edit: Those pictures don't show things too well, the number is on the opposite side of the bike at the top of the seat tube. The wheels are 26" x 1½" and the rear end of the frame (can't remember what the parts that run from the top of the seat tube to the dropouts is called) is bolted instead of brazed/welded.
Anyway, thanks for your help and everything