I stumbled onto a mission (a secret one I think?) behind the drug store in Valentine.
Go behind the store. Peek in the window and a guy is there counting money at a table. The game notifies you that the mission is unlocked.
Go around to the front of the store and rob the store. Left trigger the guy and point a gun at him.
He takes you to the back to let you in where the guy at the table was with the money.
Shite goes down as you eliminate them.
Take the cash. Open the wooden box on the table and it gives you a sweet Schofield Revolver!
Make sure to bring a stick of dynamite with you and blow the safe on the wall! More cash!!
Chest on the ground as well but can't remember what's in it but there is something there.
Beware, you are going to have to shoot your way out of this mess you made.
Great fun!
Also, you should hitch your horse a bit of a distance away from the drugstore because once the lead starts flying your horse will be nowhere to be found. I left him at the opposite end of the street where they are constructing a new building. I just had to run to him after. They will pursue you on horseback.