I have one , photos are on the forum, including some taken at Farleigh Castle this year.
The only difference I can see between a TR5MX and a B50MX is the name on the tank, if it has one, and a Triumph badged Nippon Seiki speedo , if it has one.
The engine and frame numbers are B50MX then letters and numbers just like a BSA badged bike. I have actually reverted to using BSA tank badges anyway.
Buy it and enjoy it.