Maybe a 35mm panoramic negative carrier for a 4x5 enlarger or a contact printing frame for direct duplication? If it were a contact printing frame, you'd expect at least one registration pin to keep the perforations aligned.
Wonder if it could be an exposure plate for a sensitometer of some sort?
I doubt it was cinema related, but that's only an assumption. If it were used for splicing, you would think there should be some wear somewhere on the clear plate where a cutting tool was used. Splicing tools get marked-up rather rapidly when used and I can't think of an obvious use for such a holder.
What puzzles me are the two screws in the clear plate, with matching holes to allow the top plate to sit flush. Someone went through a bit of trouble doing that, but the intent is not obvious to me. Why would you need those? Registration of some sort?
Maybe something home built for a highly specialized process. A real mystery...
@Todd Barlow - can you give us any context as to where it came from or what it was included with when you obtained it?