I think it's basically a custom hen's tooth. I could not find the manufacturer. Pic of the topside could add more info.
Modern parts, instead of pins exiting under the middle, the encoder PC board is extended sticking out and the pins come out there.
Looks like Bourns ECW but
EPS has the push switch.
Original bolts to the front panel, so the PCB doesn't give much support?
If I loved the piece of test equipment, I'd make an adapter PCB and fit a newer part (they are thinner, smaller, even slim SMT)- assuming the quadrature switches share a common on the main board.
The old part died from either wear or oxidation, so in desperation I would flood it with PPE to see if that fixes it. But it's plastic bushing and shaft so not a long life part in the first place. Or it got bashed.
Remote image loading issue appears to be due to private browsing, tracking protection: 'The resource at “https://pbs.twimg.com/media/F8dv97rbQAAEd6W?format=jpg&name=medium” was blocked because content blocking is enabled.'
This is on a few browsers I tried, unless I go directly to the URL.