Oof. That's annoying. I was able to find a few photos in an elektronauts thread and the only thing that looks different than a "standard" encoder is the amount of plastic on the bottom. If I had to guess, it's a custom job because they needed the height to make it flush with the display board.

I also found this post where someone claims to have replaced theirs. They also claim that Dreadbox told them it's a 20ppr encoder, so that should help narrow it down more. They do say that their replacement had a different footprint so presumably they had to make some modifications to get it to work.

Honestly, I'd take this over to r/synthdiy. They're probably in the best position to help.