FWIW, I had what I believe was a good semi-business relationship with Baker Audio, the local Altec distributor/system designer/installer, in that they were good about letting me know what/when the latest n' greatest was coming 'down the pike' in case I wanted to be the 'first on my block' to own it and give me a bit of a discount to boot, I guess mostly because I was on the Tech Letters mailing list.

Anyway, one such call was about a new 'HIFI' 'ultra light' diaphragm to upgrade from the 34647 I'd been using. Don't recall when the call came, those were tumultuous, life/career changing times for me, but the receipt when I picked them up is dated 2-26-76, so lacking any concrete proof to the contrary, I'm satisfied that this is probably about the earliest it was at least officially available to the general public as an 'upgrade'/field replacement.

GM