I am not sure what a stock yard padlock might bring. Knowing the back story or origin of a logo padlock will ALWAYS help so at least you have some of that. So if I was to guess I would range it $10-40 depending on who is bidding but that is a real loose guess, I don’t collect padlocks. Railroad collectors are in greater numbers as they collect locks and other stuff like date nails. Do mention that the key is marked with the railroad markings.
Your next move is to find out the history of the stock yards.