The earliest reference to an etalon in an ion laser I've found is an ad for the Coherent Model 50, their first ion laser model, in September 1967. This is a really important little improvement in ion lasers for us holographers, and I'm curious as to which laser was the first to have one. Do you have an idea when it came in?

Also to follow up on the Hughes 3052 argon laser. It was introduced in June 1968 with 100mW TEMoo (without an aperture). The price new was $9600. It was the sixth model of ion laser made by Hughes after they did their airborne devices. To date I've identified 16 other manufacturers of commercial ion lasers before the Model 3052 was introduced. I just bought a '67 Orlando Research Model 300 head without plasma tube. They were the first company formed specifically to make ion lasers, and this was their first model. Anyway, the HAC-3052 is nowhere near as early an ion laser as originally thought, but still a very rare and wonderful instrument.