You should check out the
Magnavox Friends Group on Yahoo - they have many of he old brochures scanned from the 1960's with the model numbers in there.
...okay I just looked through them and the closest I found was a 3RP656 in the '66 catalog. But Magnavox used similar numbers from '65 through the early 70's. Post some pictures of the set and we can probably narrow it down.