I have measured a Trek 720 I have acquired recently and it has EXACT same measurements as a Trek 520 touring bike. The difference in the frames between my older 720 and the 520 is that the 520 uses a seatpost collar, but I have seen pictures of older 520s (late 90's)without a collar as well.. They both have CroMoly frames. The obvious differences are the equipment, which can be easily remedied.
Does any one know of any other differences in frames besides the stickers? Did just stumble upon a deep, dark Trek secret?
Trek 720 Multitrack frames can be bought for little compared to a 520 frame.