Pretty common, several of my A-Bodies have factory Moonroofs (aka Sunroofs).
Both of my '73 Dart Sports have factory moonroofs, so does my '73 Duster, and I have a spare roof from a Moonroof car, as well as a roof from a "V3X" Car (V3X being the Canvas Roof, that one is pretty rare, since it was only available in 1971 and 1972. I've heard there was only about 60 of those made).
Many of the parts are readily available/cheap too... IF you shop outside of the mopar world. The factory Moonroofs were manufactured by Golde, a company that built Moonroofs for everything from Fords & Chevy's to Porsches and Volkswagens.
Cables can be crossed over to other applications, for example, same with the "transmission"/worm gears, crank handles, etc.
Example on the price difference. Mopar world price for a Crank Handle : $70+... Buy it for another Make/Model car : about $5. The Handle I bought for my '73 Duster was for a Volkswagen, had the same part number as the "Mopar" Handle, same style. Brand New, cost me $5