I have yet to acquire one of these World's Fair padlocks, but I guess that it one of the joys of collecting, the hunt. There does seem to be a bunch of these floating around. More common than the 'Empire City Subway' padlock that I have only seen on eBay once.
Up for sale is an excedllent antique brass 1904 Worlds Fair ,St. Louis ,U S A ,Eagle lock Company 3 1/8 ". Lock is in exellent condition ,no key - eBay (20 Nov 2011)
This padlock had a high bid of $464.59 (Plus $5 Shipping).-----