[CAL 58 GMC]

Hi and welcome to the board! As for how rare is a small back window, well the 5 windows was considered the "custom cab" and would be more desirable then a small window. The same holds ture with the 1955.5 to 1967 large back window cabs. The small back window was the standard cab. There are some people who rather have a small window over a large, but when it comes to which is worth more, then the 5 window or large windows have more value to them.