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I have a '68 and I currently have every last nut, bolt, track, you-name-it out of both doors. I'm rebuilding everything inside, so I'm very familiar with the inside of a '68 manual window and locks.
I can't think of much that would allow it to move forward/aft. There are large plastic washer/bushings where the bolts go through the glass that might be broken or missing. this would allow the window to move some. The vertical tracks may be loose or the rollers that ride in the vertical tracks may be damaged.
There is a large black metal access that can be removed and you can get to most of the lower part of the window.