Every complete bike Trek built until '79 or so was built to order -- even the 300 series. You can see the racks of completed frames in the famous picture of Mike Appel and Dick Nolan in the early brochures. Framebuilders would build batches of 20, 30, 50 identical frames. But when a bike was sold, we would pick the frame and it would get any braze-ons and then painted and assembled. It was always my impression that some of those frames sat on that rack for longer than they should have because of the way they were built in batches. So that's why a bike may have been sold much later than you would expect from its serial number.

Fun times.