Barnfind,
First year the "factory" power sunroof was available was on 1969 Chargers. I put factory in quotes because the sunroof was installed away from the factory at American Sunroof Corp's facility. The sunroof was available again on the Charger for 1970, along with the new 1970 Challenger.
In 1971, it was expanded to several different models. Charger/Superbee and Challenger, Satellite/RR/GTX, Fury line, Newport/New Yorker/300, and Imperial.
In 1972, it was expanded to Cudas/Barracudas.
M51 is the option code for the power sunroof, but you will not find it on most 69-72 fender tags (there are just a few exceptions). Most of the time you can identify a factory sunroof car by the VON on the fender tag. Depending on the year and make, the VON on the fender tag will start with a specific number or letter/number combination.
If you have any other questions pertaining to sunroof cars, send me a PM and I will try and answer them.
Scott.