The negative was a film neg not glass.
I've seen that before in 'old bits in a box' that I couldn't identify, the film adapter (for smaller film) looked like a professional job as I recall as there was a name on it (what I can't remember).
Originally Posted by GregW
Here is a terrible flatbed document scan of the "adapter" . steve, that's interesting that someone marketed something as simple as this. I went ahead and inverted the scan and darkened it so you can see the old image on it.
Attachment 74851Attachment 74852