Originally Posted by
Classtime
Is this 660 from the years Trek used the one piece head tube with lugs?
Yes. Lots of tells in the details - look at the faux cutout in the lug extensions, then the thickness of the lug where it meets the top and bottom tubes, compared to where it meets the head tube - the head tube junction is much thinner - this is where a press punched it out to make it look like 3 pieces.
My '83 Trek 600 has it. My '87 560 has it as well, which sports the same version as the picture you quoted. I don't know if they used it earlier than '83, or if they also used it on higher end models of the time period (i.e 7xx).