Thanks for all the input. jdefran I PM'd you. Also, The Golden Boy and Road Fan, I didn't mean to spark up some previous "thing". I'm clearly less knowledgeable, hence the amount of questions and disclaimers in my original post.

Also, Golden Boy, if Trek didn't want 6xx to denote anything in particular, why did they make different bikes called "6xx"? or "4xx" for that matter. I think from the old Trek catalogs I see the second numeral identifying the type of bike (x1x is sport, x2x is touring, x6x is race). So if the first numeral (6xx or 4xx, etc) didn't denote tubing, what was it for? I'm probably wrong again, and probably my comment will spark renewed debate.

I just like the chestnut color..... Any chance that somebody wants to answer my question about which years Trek used that color? I see 1981 and 1981.5 as the only model-years that included this color - according to vintage-trek.com.