Steel 120 with small neck threads

[Leo53214]

Hi guys-

I just bought a used steel 120 in great shape, EXCEPT (of course there's an except!) the valve knob just spun completely around. Turns out the previous owner must have broke the handle, then drilled it out and shoved a screw in it. It is a Genysis DIN valve and though I might be able to fix it- I'd much rather just replace it for safety.

Here's the catch- the tank has a seat at the neck and the threads are smaller diameter than a standard valve. I was thinking maybe an old 1/2", but did they have Genysis DIN valves in that "era"? The really frustrating thing is that I just switched everything over to DIN H-Valves and now I don't know what to do with this tank if it can't follow suit.  Any suggestions?