They are rare, I can't tell you the production #'s on them, as I don't believe there's any accurate production figures on these. The cars were shipped to ASC (American Sunroof) and retro fitted there.
The canvas fold back are actually rarer IMO, as I have only ever seen two.
There are a few members on here with Dart Sports and Duster sunroof cars.
I have just installed a sunroof roof skin into my 74 duster. It was from a rusty 74 duster I stripped out years ago. I have seen afew cars over the years. Do a search for sunroof on the forum, as there was a thread on that about a year ago. Lots of pictures.