If the window doesn't roll completely down, then the regulator is a standard 1968 Pontiac part, which might also fit backwards to 1967, and it is anyone's guess as to how far forward. It might have been modified by the coach builder, and if it is just a roller that has come off the end of the arm, then that can be replaced. There are more than a couple types of rollers, so you will need to check to see what type you have. If you take the door glass out, it would be best, since the glass is tempered, and it can break quite easily. If you are not familiar with this process, take it to a glass company that is familiar with old cars, and have them take the glass out, along with the regulator. They might even have rollers in stock to do the repair. If you break the glass, and it is a propriety piece of glass, then it will be a long time before you will ever find one, if ever. Err on the side of caution.