Yes Waldo, from what I understand the later Mark 1's were the first to use the Fetron's ....The Mark 2a's and some 2B's were the last to use the Fetron.... so as you can surmise the Fetron was a pretty short lived experiment. In fact the way they sold the amps back then with the Fetron's is that they shipped the amps with the Fetron installed but also included a nylon clip screwed on the inside of the cabinet that held a 12ax7 as a replacement choice for the Fetron. If you ever see an amp for sale that still has that clip to hold the spare 12ax7 you know you are getting an amp that most likely has been taken care of! Mine didnt have the clip but had the Fetron. It was only around for a few years.
As far as if Jerry's Mark 1 (and by that I take it you mean the Mesas he played in 75-76 before returning to the Twin in Oct 76) I would say that no it did not have a Fetron. I think the Fetron started sometime in 1977. You never know though. They always got their gear off the production lines faster than everyone else!
And to add one more edit
........ The 1980 amp definitely had the Fetron. And from the sound of that Winter 80 tour I think he chose to use it over the 12ax7......
.......................................................have you heard the one about the yellow dog?