It is not a full Reynolds 531 frame. The main triangle is reynolds 531; the fork and stays are reynolds chrome moly (probably 501). The info is on the Trek website.
This is a kick ass bike. I have one and it is a classic Trek sports touring bike. It can fit 32c tires or 28c with fenders. I run it with a triple and lightweight 32c tires (clement strada LGG 700 x 32c); the bike is a real mile eater.