Well if it says yale on it and has eaton on it then there is no question....it is a yale :o)
Now as for the serial number location there a couple places to look for it. First place would be on the frame right around the right side step area, it would be stamped either on the top beside where the floorplate edge is or on the side of the truck frame in the same general area.
Another place to look would be on the front of the cowl on right side along the bottom edge between the mast and cowl area.
A third place might be along the right side engine compartment along the top about where the edge of the hood is.
It sould be either a 7 digit beginning with a letter... or a longer number at least 13 to 15 digits, starting with a letter like G and ending in a letter that could be any letter.
I also should tell you they did not stamp these sometimes very well so it may take some scraping, if it has been painted then that just makes it more fun to find :o)