World War II theater-made knives with Plexiglass handles definitely exist and can be worth some money to the right collector. A few examples:
The problem is, there are a lot of fakes in the militaria market. Without provenance and/or documentation, you have no way of knowing when your knife was made.
Perhaps the best you can do is try to get hold of the authors of this book to see if they can help.
Identifying an old, unmarked, handmade knife is extremely difficult. Yours seems very well constructed, though, particularly the sheath. Maybe someone in
Bernard Levine's Knife Collecting & Identification subforum has seen similar work. Good luck!
-Steve