Thanks guys. It's not a KB-5191. Since I posted this question, I was able to identify the keyboard as a mysterious Chicony KB-5391:
Unfortunately, there is absolutely no info about it on the internets. Someone on another forum suggested the switches might be Deyi foam and foil. It is indeed very similar: the housing is identical, but the slider is a little different. Whatever the switch actually is, I agree with @sup that it probably is a foam and foil mechanism.