I spent over 30 years in Ford dealerships. I did everything from being the wash boy to being the dealer. As Karl mentioned there was a P/N prefix, base number, and suffix. The 60's,70's,80's numbers were fairly easy to recognize just by looking at the number on the box. The third digit(letter) of the prefix told you what model. A was full size, D was Falcon, Maverick and later became Taurus, O was Fairlane (Torino), S T-Bird, T truck, U Econoline. As Karl said many of the base numbers never changed. 11654 as an example is a light switch. The suffix does matter. At one of the first dealerships I worked at they told the new parts guy to order 6 sets of pickup fenders. We ended up with 6 sets of the original size Bronco fenders. I'm sure they'd bring a fortune today as NOS. Sometime in the 90's they went to "Global" prefix's which I never really grasped.