what was the first year for the space duster?

1973 was the 1st year as stated above.

Fold down rear seat with trunk dress up option C73 was known in the brochures as the Spacemaker package

Chrysler nicknamed Dusters with C73 option and M52 manual sunroof option as "SpaceDuster". The Dodge Dart Sport version was nicknamed the "Convertriple". In any Chrylsler literature I've seen when the term SpaceDuster is used the C73 & M52 options are both pictured or mentioned together.

Many 1973 sunroof A-bodies were made with just the M52 sunroof and not the C73 fold down seat option and vice-versa of course.

My old Duster was a 1973 340-4spd car with M52 and C73 options besides many other options.