All cars after 1966 got tandem master cylinders.
It sounds like your car may have the later A-body calipers (1973 is first year), that would be a reasonable match with the later rear brakes you have.
You can do a search here for 1973 Master cylinder part numbers.
Here's one I posted: How come all my manual drum cars have disc master cylinders?
There will also be a number cast into the bottom of the m/c. The article on master cylinders archived at moparts has many of the casting numbers cross referenced. http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf
