Long story short, my Father in Law ended up with Marconi Instruments Ltd No 1 Mark II* signal generator and a dozen boxes of assorted parts. I’d guess it was an Elmer‘s Junkbox.
Can’t find any information on this on the web. Marked IN 2631 and SI 7545, my best guess is that it might be a solid-state rectifier intended to replace a vacuum tube.
So if Google brings you here and you know more… that’s what the comments are for.
Edit: It’s a ONE N 2631.