I finally got around to disassembling the Rubinette for repair. It turns out to be a perfectly straightforward button filler, and there were no internal clues to its origin.

 

It scrubbed up pretty well, apart from some ink traces on the barrel threads which I’ll remove later – I have my methods! Further extensive searching online and in my reference materials found no reference to this pen nor any tie-up with the 1950s Rubidor ball-point.