I did a little digging and see what you are talking about. This was the North American "small pane" versus "large pane" canopies with the T-6G and the Harvard. The large pane version is called "reduced frame" and appears to have shown up on later models of T-6G and Harvard versions.
According to Boeing, 15,495 T-6s and their variants were built. They were used by 34 countries over at least 25 years, so I don't really know how many versions, and variations on the versions there are, but I am guessing that it is a big amount.
The large pane canopies seem most common in Southern California, and I believe, but will have to check that they are pretty much interchangeable. I would doubt that you would have versions that would only take one type of canopy over the other.