- looks like it really must have been a demonstrator of some kind. I have a Russian Boctok (Vostok) with also an unusual case in black plastic, never seen another, as you will know all Vostok cases are normally stainless and the Amphibian range is water resistant.
and this is it - a manual wind which reproduces the standard Vostok case in black plastic. The watch has a stainless back fitted with screws.
So it looks like Timex beat Swatch to it Bill with disposable cases from plastic, and maybe so did Vostok. Never a truer saying "Nothing new under the sun!"