This is the inset of a Cherry M8 SPST key found in (among others) the Epson PX-8.
I picked up one of those with a broken key so I designed and printed this replacement.
Depending on the state of the key holder of this inset you might want to file/sandpaper some to make it slide better.
Remember to cut the pin on which the spring is seated because I had to extend it for support while printing.