Don't get me wrong Ian, but I have never ever known a woodworking tool with, firstly 'morse' engraved on a STEEL handle. Secondly, centre drilled both ends.
Yes you have your grandfathers tools in your pocession, not disputing that, nor your 55 years experience.
It is a plug gauge, period..........40 years engineering experience! How many times did one trade 'borrow' another trades tools, plug gauge has hardened surface, very useful to sharpen things on.
I am saying no more on the matter.