First clamp it in a vice on the 2 flat sides below.You can dismantle it by pull out the black knobb/button.
The first 5 or 6mm will go easy.
Than it will look like it is stuck.you need a pliers to do the job and pull harder.
The knobb,a spring and a inner clamp ring will come out.
After that look inside the housing.
You will see a milled cross at the bottom with a hinged bar in 1 of the lines.
This whole piece is a threaded plug .
Screw it out...not always easy...
When it is removed a ring will be visible..it can be realy stuck inside the housing..in the housing is one spline (give it a good look) and on the ring is a spot to secure in the spline after that the membrame is visible .it is the last piece inside..
Roy's instruction how to dismantle it is accurate. I have exactly dismantled 4 of them in this way. Below a photo of all the parts which come out of it, excluding the membrame which was in very poor condition. Usually the old membrame is causing oil leaks with these indicators, I always put a new one 1 when I am working on an early BSA.
Thank you both. Really helpful -I'm glad I asked the question before reaching for the toolkit. The component parts look less complicated than the technical drawing.