Late 70's Trek

Go to http://www.vintage-trek.com/SerialNumbers.htm#prior. They have tips on getting the serial number, and even if you only can read part of it, you might still be in luck. Unlike later Treks, where the number is just a number, the '70s/'80s bikes had numbers that told you the model, size, and year of manufacture. So even if you can only read part of it, you might still be able to figure out the year. The site also gives some guidance on getting date stamps out of your components (your derailleur, for example, almost certainly has one).

For my two cents, I'd say you have a late '70s model. The key is the brake-cable clips on the top tube, which are only on really early bikes; in the early '80s, I think, they went to braze-on cable guides.