Usually they're listed as coupe, convertible, and sedan. I've never heard it
listed as fastback or sportsroof, which would make WAY more sense, duh! They
are definitely NOT the same and the Sportsroof windshield ONLY fits the
Sportsroof. No other windshield will fit. I had the windshield replaced on
my '69 Sportsroof and IIRC, the part used was actually listed as "sedan". If
the glass shop you're buying from thinks it's all the same, then go
somewhere else. The Sportsroof glass is not as tall and has more slope than
the coupe or convertible glass.

Call some of the Mustang parts vendors (most have 1-800 numbers) and
some of them list the parts using the original Ford Motor Co part
numbers.

I would suggest:
www.karmustang.com  which does actual restoration work, not just sell
parts.

There are a couple of others that actually do physical resto work, but
I don't have names handy, but someone else may....  

Hey! Spikey Likes IT!
1965 Ford Mustang fastback 2+2 A Code 289 C4 Trac-Lok
Vintage Burgundy w/Black Standard Interior
Vintage 40 Wheels 16X8"
w/BF Goodrich Comp T/A Radial 225/50ZR16