Not as hard to come by as you might think. Pre unit Triumph Trophys through late '56 used a 20" rim with 40 holes. Also the BSA B50T and the B50MX, as well as the B25T and the T25T Triumph (which was really a BSA with a Triumph badge) I think, used a 40 hole 20" rim. Those last bikes are from the early '70's. Someone on this board offered me a complete wheel from one of those just the other day. I will pass on to him that you are looking
Although it's not for sale, I scored a flanged alloy 40 hole 20" rim somewhere. It seems too light to be a Vincent rim, and from the weight and dimples looks like a SanRemo brand. You may already know this but every once in a while Avon tools up and makes a batch of 3.00 x 20 Speedmaster MKIIs. I have one to go on that rim.
Oh ok I didnt realize there was that many bikes that came with them. I havent seen any in the past few months. My plan is to use one of the 20s fitted to a star hub on the rear end. This rim doesnt have to be perfect or some crazy vintage part. Just looking for something straight and runable.
And that would be great if you could pass that on to the other member.