If in fact your brother is Mike in BC Canada, then he visited me here in Oregon. I showed him some of my BSAs and piles of junk. I have a early plunger golden flash along with the parts I have to make a " Hot Flash".
He kept going on & on about the triplex chain and I tried explaining I have seen a few and in my opinion,
(Which could be wrong) The triplex chain setup is not that unusual. He insisted it was but what stopped him in his tracks was my bike sitting here, has a triplex chain setup.
I have sold a lot of parts when I had my shop, and can tell you I got a number of requests for parts from these semi unit bikes, Here what people want, to convert a Harley 45 to a British transmission.
A) BSA semi unit mainshaft.
B) BSA Triplex clutch setup OR, Norton Commando
C) BSA later PU Gearbox & Some other British bikes parts, I can call some people to refresh my memory, but its well known
(The HDs are very limited with their 3 speeds on the HD flatheads, so very popular to switch to a British 4 speed tranny and BSA is a common swap. The 3 row chain setup lines up and works with the HD engine.)
Any rate,,, if its hard to get parts for why not upgrade primaries? Many people switch to the Triumph clutch baskets, or better yet put a belt drive setup on there and be done with it?? Several vendors offer belt drive kits and while I have yet to run any on a BSA, I can say from most Nortons and Triumphs I have fitted them too its a massive improvement, Elements the oil leak and sticking clutch issues, weighs less so less wear and tear as well as a noticeable boost in power.
Once the primary cover is on no one is the wiser.
Congrats on finding a bike!
EDIT: I confused posters, and now see you and Mike from BCs Brother are 2 different people, My bad! Sorry for the confusion.