The GTX and GTX pro he used from adidas was graphite/fiberglass......and it was not a midsize....like 70sqin. He started using a midsize frame at the end of his career and at Wimby when he was desperate to win a title.
I'm sure some of the GTX adidas experts here will chime in with exact specs. I first remember seeing him at the orange bowl in the late 70s when he was using if I remember right a Maxply Fort....still had a HUGE forehand! Solid serve and his backhand got better and really strong after 1982.....had a wicked slice as well.