Thanks - never seen that picture before. Easy to tell that cars been messed with allot. Must be some sort of experimental PB unit and something else on the drivers side radiator support,
Guessing you believe that the air cleaner is original to the car (rust patterns match everything else). If so I agree with Charles you need to look for a pattern around your cars build time and plant. We've got little idea of when the car in the picture was built - Looks to be a NJ or Dearborn car
It's an interesting article from the February 15, 1968 edition of the English magazine Autocar about a company that converted a 65 Mustang to all wheel drive for use on icy roads. Obviously, the car had some significant modifications, but it seems like the air cleaner remained stock (as another point of interest, you can also make out the alternator suppressor capacitor on the left hand side of the engine)
I agree with Charles' suggestion, but it is a challenge to find good examples of stock decal placement. Restored cars are not good guides, because almost everyone installs the FRONT decal which is usually included with the decal kit (I've even seen the FRONT decal on six cylinder air cleaners, which we know is incorrect). So, that leaves only unrestored cars and period pictures to serve as examples, and we know there aren't many of them.
My admittedly incomplete research suggests the FRONT decal was mainly used on late 65s into early 66. I've never seen one on a 64-1/2. I haven't been able to identify whether there were different practices among the plants, but I suspect the car in the 1968 magazine article might be a Metuchen export model (note the export brace and English license plates). If so, it might be a good parallel to my Metuchen export car.