The GM 72 carried model numbers of 30778 or 30780. They were produced from the late 70's to mid 80's before changing model numbers and names. There should be a model/serial tag mounted on the sheet metal below the seat about mid-calf on your left leg. If all you see are 2 small holes (where pop rivets were), that means the tag is gone.
A Service Manual is available at www.Toro.com
and can be downloaded for free. However, If you desire an Operators Manual, that will need to be ordered. You can do that directly at Toro.com also.
The Toro Company