There were several models of Fender Rhodes 73 that came out starting in about 1965 and ending in about 1984.
The models with the accessory 1 and 2 jacks first appeared from 1975-1979 and are called Rhodes Mark 1. In late 1975, CBS Musical Instruments decided to drop the Fender name from the Rhodes line of products, in order to establish Rhodes as its own brand. The Rhodes Suitcase Piano and Rhodes Mark I Stage Piano continued to be produced in 73- and 88-key configurations.
The accessory 1 and 2 jacks were first only found on the Suitcase model - the one that sits on top of its own amp. The Stage 73 from this period had a different namerail and controls than the Suitcase model. The Stage 73 could be retrofit and have stereo tremelo, however, with the same namerail and controls as the Suitcase by purchasing the Janus Stereo amp upgrade.
David