Well, what's important to you, going early, or going built-in multiplex? I have a Harman Kardon FM-100 that is a copper-faced beauty, and it was introduced in 1956. It does not have a built-in multiplex decoding circuit, but they were anticipating it...there are two RCA output jacks, one for mono connection to a pre-amplifier's "Tuner" input, and one that bypasses the de-emphasis circuits for connection to an external demultiplexer. I have a Fisher MPX-100 (from about 1960) that does admirable duty in that regard.
As for stereo stations, here in the SF Bay Area, KPEN is pretty-well known for breaking ground in stereo broadcasting, under the stewardship of local-legend Jim Gabbert. They turned on the first stereo transmission west of the Mississippi in July 1961. You can hear some vintage air-checks at:
http://www.bayarearadio.org/audio/kp...959-1960.shtml
--Bob