My 77 has the alarm on the drivers side fender which makes it an early.  However, when I counted the pins on the T-tops when I ordered new weather stripping I counted 19 pins which would make it a late 77. I was told that at the factory they would use whatever was on hand and that's so true when it comes to my car. Play it safe and count the pins.

The assembly instruction manual (AIM) probably has a revision date on the page regarding the weatherstripping on the tops.  If they changed from 18 to 19 pins at some point during the model run, you can bet there is a revised part number in the AIM and a notation as to what changed on the revised page.  Use the revision date and your time\build code for a comparison.  AIM also shows how the weatherstripping installs.