Complex sentence: The Tribsa, an alternative to the Triton, was a Triumph engine in a BSA frame but other frame and engine combinations were also made, with Norbsa having a BSA engine in the Norton featherbed frame.
Split into simpler sentences: The Tribsa, an alternative to the Triton, was a Triumph engine in a BSA frame. Other frame and engine combinations were also made, such as the "Norbsa" which had a BSA engine in the Norton featherbed frame.