Here's one, on a mag (off an AMC twin). But the stop button has been replaced by an insulated sleeve, brush and spring, for remote stop. Which may provide a useful clue if it's this sort in question. Originally, there was just a mushroom-headed bit of spring-loaded bar, that shorts out the live cb centre screw when you press it. Not handy, some say (including me), hence mods.
There are plain end covers, held by a pillar and clip, with no take-off for a wire or button even - but they usually live on K1Fs which people stop with the decompressor.
The three basic standard K2F versions have provision, with a wire off the plastic covers retained by pillar and clip, or via a stud connected to a brush and spring in the middle (big screw-on alloy type) - or there's the button sort.
On BSAs I think - but not completely sure - that the alloy dome was used on some twins in the early-to-mid 50s, before going to the clip-on plastic sort with wire off, and then finally to the screw-on alloy job towards the end.