67 Small Rear Window? Whats the story?

[68 C-10 Panel]

Hi guys, I drove an hour today to go and look at a 67 c-10 pu I found on Craigslist. The truck is in pretty rough shape but it could be saved. Long story short I would like to tap the great knowledge that lurks round these parts. Whats the story with the "small window" trucks. Are they uncommon, special in some way? I looked at the spid in the glovebox and the list was pretty short, nothing to special. Except it said something special about the heat, heavy duty or something like that. Thanks in advance.