My understanding is that Rossin left his position as chief framebuiler at Colnago in 1974, to form his own company. His tenure at Colnago reportedly goes back into the 1960s, but I can't say exactly how far. I've read that he apprenticed under Poliaghi.

I agree that the definition of mechanic has changed or at least blurred in the translation. As previously suggested, most of the head mechanics for the pro teams were actually repsonsible for the frame construction. In fact one of Colango's tenures as assistant mechanic was under the tutelage of head mechanic Masi. Personally, I have little doubt that Colnago actually built frames, though just how long after establishing his own brand, is open to specualtion. Colnago's own literature mentions him apprenticing under Funagalli, then later moving on to Gloria where "he sharpened the art of welding".

As for the Merckx association with Colnago, that was 1971-1973, which is late enough in Rossin's tenure at Colnago that the later frames were almost certainly built by Rossin, if not all of them.