Hi Mike,
From your photo I cannot see if there are cooling fins on the hub ?
It looks to me that the hub fitted is a modified non finned type that was fitted to M21's and some early 650's, B33's??
These hubs are steel rather than the cast Gold Star type, normally the GS type has more smaller holes in between the large diamater ones
The "proper" GS hubs have 1 1/4in. wide shoes, later very similar hubs have 1 3/8in. wide shoes these were fitted to A65's and some B 441
The 8 inch single sided finned hub is regarded as the best brake that BSA made
There were period TLS brake plates made for them, rare enough now and much sought after
I will try to remember to photo the finned hubs tomorrow as I have both types in the workshop
If you wish to fit the 8in. full width hub and Triumph type TLS brake you will need to change the fork sliders and maybe the mudguard brackets, the anchor slot/ lug will need modifying too
HTH
John