I decided to remove both bottom members, because there was some leakage of fork oil from the left one.
Dismantling was easy. I took the wheel out, mudguard off, then unscrewed the allen-headed bolt in the bottom member that is concealed by the axle. That allows the bottom member to drop right down, end exposes the top of the seal holder. Unscrew the seal holder, and the bottom member slides off leaving the bushes and seals in place. I used a bolt in the mudguard mount boss to turn the bottom member while I held the seal holder with a C spanner.
Assembled the wheel with the Trumph backing plate in place of the BSA backing plate, and mounted it up. Looks good, no interference with the nave plate, nothing to take off or modify.
Problem is that the brake drum rubs on the inside of the backing plate. I'll have to take the wheel out again to dismantle it and fit a shim for clearance. It's only just touching, so it won't need much.
And I messed up modifying the brake cable to fit. The outer needs to be longer, and the inner needs to be the same length as before. After my efforts, now the inner is so much too short that I can't fit the cable.
My local bike shop is closed today, so it will have to wait until Monday night after work. Fortunately generic brake cables are cheap and plentiful.
Even though it's been three steps forward and two steps back, it's been a good day. It looks like it is all going to work at last.
I have the correct lower mudguard stay, but it doesn't fit nicely with my late model mudguard, so I am keeping my handmade one in use until I can score a correct guard.
I also scored a right hand choke lever (it's in the post), but I suspect it's for 7/8" bar, so it will have to make the acquaintance of Mr. Dremel, like my left hand lever did.