A number of older threads where the cost of these frames was discussed often took inflation into consideration, as it's more meaningful this way for a contemporary comparison. A $140 MSRP in 1975 was equivalent to over $600 ten years ago, now it's over $800. The mail order price for the C-6 came down to just over $100 in 1977, which was indeed equivalent to about $400 ten years ago.
If Tony Trabert qualified as a pro, then he was probably the best known pro user.
Joking aside, no self-respecting pros would touch these sticks in those days, as they were considered a) a fad, and b) a crutch for the unskilled. Only those who had a lot of money to burn and relatively little confidence in their own strokes would give these new Wunderwaffe a second look in the beginning, which typically meant middle-aged doctors, lawyers, finance guys, and their spouses. The highest profile (and very typical) user of the C-6 was probably Walter Mondale, who was documented to have wielded one of these during the 1977 celebrity-studded RFK tournament. Any pro who showed up with a C-6 in hand at that point would probably have elephant hide for skin.
I have been told by the people who went around the country selling these early graphites through club events that women tended to be more responsive to these pitches than their husbands, because some of the latter were too macho to admit that they could lean on technology for help. As a pro yourself, I am certain you have witnessed this type of dynamic.
This is one major reason being first to market is not always a winning formula for new tech - as it can take time for the market to realize that it "needed" the product.
By the time this tiny ripple became an overwhelming tsunami, as documented in Grafil's thread, the original leading edge had long been rolled over, buried and forgotten by most, if it had been noticed at all in the first place.