Hi Craigy, you have the plunger model with the (cow pat/divers bell) nacelle, you are in luck as there is quite a few crono speedos of varying face numbers to suit you for that bike, it is everything from early A7/10 & some B models to the last of the swing arm A7/10 and B32/34 even gold stars, the main difference is final drive (gear box sprocket/wheel sprocket), hope this doesn’t razzle you too much, but they are available.
What you do need is a unit that has the trip reset poking out through the bottom of the casing, then there is a long plastic extension that gets connected to that via a split pin ( more on that later).
The chrome bezel for the cow pat nacelle is unique in that it has a small protruding lip, this sits in a unique rubber speedo mount grommet, to get this speedo into the rubber and then into the cow pat nacelle without cursing all of the BSA design team is NOT possible.
So some good points, and some challenges.
I do have a great chronometric guy in NSW that will work with you on whatever you can come up with, I may even have a correct bezel, just gotta look for it.
Thomas.