There is a metal channel on the back of the door that's held in by a couple of screws on the '51. If that isn't in place, the glass will be loose when it's rolled down. Their catalog is in error, as that's used in the later trucks as well. It goes in opposite the division bar with glass channel on your vent window assemblies.
It's shown in the FAM on Section 1, sheet 22.00


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.