Thunderbird Films was the company that was selling 16mm prints of first and second season Star Trek
episodes. They won the suit filed by Paramount.
During that time, the first two seasons were effectively in public domain, since the copyrights were not properly transferred when Gulf & Western acquired both Desilu and Paramount and dissolved Desilu - merging both studio and assets into Paramount.
After losing the suit against Thunderbird Films, Paramount was later able to reacquire ownership and copyright of those episodes in 1978.