I think 520 is the standard, so that lighter & heavier weights of the font are either lower than 520 or higher etc... So for people using it and unsure what standard is they go with teh 520. If that makes sense?
caslon was the classic font for books created in 1700 and something by William Caslon, There have been countless variations and modernisations of the font.
Caslon 540 was created specifically for advertising by the American Type Founders and was in fact a variation of a previous adaptation 470 - or some number like that. It has shortened decenders and has no bold wieght. Became more popular in the 60's with the advent of Letraset etc.
Caslon 540/520 was the first typeface i was taught to hand draw, and is one of my favs.