Aha!
you've fallen foul of a factory change. The early 100 Trail and TMX models used 19mm bars from the 90 Mountain model, with BS90 switchgear and levers modified to take a front stop switch. You cant swop and use a 90 lever, though - for some reason, Bridgestone modified the shape size of the lever cutrout on the BS100 version.
Late model TMXs (1970 and 1971 years) used 22mm bars and switchdear taken from the BS175 Hurricane.
Not much you can do to modify the small diameter switchgear and lever to fit your bars, I'm afraid, but the good news is that the same switchbox and levers were used on all the 175, 200 and 350cc Bridgestones, which gives you a better chance of finding the right one.
Graham