The plastic shell with mine is just a sheet of plastic that rests on the keyboard; I use it on my AT102W instead. (The keyboard I meant earlier when I said AT101W!) Filco keyboards come with these, too, as do several other brands.
As for the Beltron variant — my OK-100M is now 25 years old, so it's no huge surprise that information on those is scarce. Some Far East brands are still around to this day, such as BTC, but most of these brands just dissolved away into nothing. All that's left of them is the odd keyboard here or there.
Bob Tibbetts has one like yours (2/3 the way down the page):
http://www.northgate-keyboard-repair.com/
Here's another Beltron keyboard, a TK-M501C using Chinese buckling springs, in the topic called
What's the worst keyboard you've ever bought? Warnings for keyboard buyers!
Yeah, so, whoever Beltron were, they didn't seem to be aiming high!