Any idea what this is for?

Hi, anybody have any clue what this was used for?
It has a cranked shaft ending in a sharp point with a sliding, pointed blade element. I thought it might be a scribe of some sort but there's no way of securing the blade bit in a fixed position so not very useful if it is. No makers marks so could be a homemade tool. Came from my grandfather who used to run a ironmongers store so no clues there unless it was remaindered stock because nobody else could figure out what it was for! An hour trawling the Internet hasn't found anything similar. Latest thought is it was for rotating inside your beer glass to leave a mark and make sure nobody drinks your beer while you're taking a leak!