73 or 74.

- Black shifter covers showed up in 73. Coverless Suntour Power Shifters showed up in 75.
- Your TTT stem isn't original.
- 72, 73 and 74 speced a Suntour V RD, which you have. 75 had a V-Luxe.

I vote 73 or 74. The difference may be so subtle as to be only discernible through components date codes: http://www.vintage-trek.com/component_dates.htm#suntour