Hi Bruce,
If the reset knob is at the back of the speedo, then its not quite the right model for your bike,
(as far as I know)
Non nacelle BSA's have the reset knob coming out at around the 5 o clock position pointing towards the rear wheel,
The ones coming out the back are for nacelle fitted bikes, these have a long piece like the outer of a control cable attached to the pin, I think its only forced on over the pin (wound)
the other types have a tiny splitpin holding on the knob.
If the bezel is stuck on, carefully wrap some double sided adhesive tape around the bezel, wrap a length of webbing strap around this (bike tiedown strap) and hold the two ends in the vice
a flat bar with two suitably drilled holes for the mounting stude can be used to unscrew the speedo from the bezel
without damaging the bezel or case!!
Beware there are lots of tiny bits in there
It takes quite a while to become familiar with the workings of a chrono
Best of luck
John O R