I also had one of these some years ago, I never found out what it really was for. It looks a lot like the one used on pre and early war BSA's fitted on the engine but these didn't had the thread on the end. The one used in the tank panel of the M24 and quite a few other BSA's is very different.
I could use that thing if you have no use for it, the inside is the same as some of the other indicators and I could use these parts.