OMG. Your're the only other person I've seen so far that used one of these. I used an MT/ST in 1972-3 at Ft. Monmouth NJ (Army). Those were indeed monstrous, state-of-the-art, hugely expensive, and rare. I didn't know it did proportional spacing, though. Mine didn't.
Actually, the MTST didn't do the proportional spacing. Running the MTST tape on the Composer was where the magic happened. It did great work -- beautiful.