Originally Posted by B. Carfree

Go here: http://www.vintage-trek.com/SerialNumbers.htm to use the serial number to determine the year of production of older Trek bikes.
Good luck finding a 720, they have quite the following. My wife and I both tour on them.

All of the older Trek bikes have one real weakness: where the seat stays attach to the seat tube. They just didn't get enough metal in there. It's pretty, but it is the most likely place to break. Check for cracks regularly. If it does crack, no worries. Just take it to a framebuilder and have him/her replace them with a stronger configuration. It's a good time to get better spacing in the rear dropouts too. I paid about $150 to have that done on one of mine.