Help identify a very strange sliding rear window??

[chevyblue72]

Hi, I'm trying to identify a rear 67-72 slider. It is exact dimensions to other rear sliders I have however this one sits in the rubber seal flush to the back of the cab and has a two piece frame. There is a full size interior trim piece that screws on the after you install the window. My thoughts are that it was meant to install after cutting a small window cab larger where you wouldn't have the pinch weld, or could it have been out of a C30.. not sure if they were different. I've searched google and forums for a month without any luck, if anyone has ever seen a two piece frame design it would be a great help. I just don't want to take the time restoring the window if it can't be used. Thanks!