Do you have the assembly manual, that should show the details to determine.
Did you look at the MPC? free download of the 75 MPC -
http://squarebirds.org/Manuals/1965/1965-72FordPartsTextCatalog/The index on page 5241 (on my "paid for searchable version" which might differ slightly from the free version) lists Mustang front door window parts as diagram P-6816 (page 5269)
It also lists Mustang quarter window parts as diagram P-6732 for fastback (page 5369), P-6733 for coupe/convertible (page 5370)
Based on that it appears there is no difference, but I'm no 69/70 expert. If I steered you wrong someone will chime in.