Can anyone throw any light on this teleconverter?

I acquired this with a job lot of filters but I've no idea what it might fit. It's obviously a Stitx 2x converter screw fit but for what I don't know.

Is it of any use to anybody? Wouldn't it be nice if it was of great photographic significance and would fetch the price of a new K3!!

It looks like a Pentax Screw fit, 42mm. This is the fitting that Pentax used before the bayonet K mount was introduced in 1975.