This is definitely something made by Alps in the 1980s or very early 90s, with dyesub keycaps.
My wife guesses it’s maybe from some kind of dedicated word processor? Doesn’t look like a general-purpose computer keyboard.
The switch is some kind of integrated dome. Looks somewhat similar to this one:
But a plate mounted version, and looks like it has pretty low travel, considering the short amount the slider sticks up.
For posterity, images rehosted: