I'm working my way towards collecting an original 60s Sony ES system...
I thought the ES line debuted in the late eighties / early nineties. In the mid seventies I worked at my uncle's shop and one of the lines we carried was Sony. We pretty much carried the complete line, and I don't recall any of it having the ES designation. If some of it did, then it didn't register in my 15 year old mind.
Did the EL Cassettes fall into that bracket? They weren't exactly popular, but I own one for a short while.