Re. the Astro-Sonic mid 60s, the AUX RCA Connectors are on the chassis right next to the RCA connectors for the phono, inside the cabinet. You'll need some nice long cables, (and you might feel like you have stood on your head to find them) but the protocol is standard "consumer" line. Mod...

I have a 1ST663S dated last I checked 1963. Magnavox first introduced stereo FM in 1958. There was no FM stereo in 1949. Hope that helps. I actually have the same stereo adapter, permanently installed by the time my model was manufactured. Anybody have calibration procedures for the multiplex adapte...