22 is Charlotte, I believe. And the other four are some sort of "special order".
Found this on the Ford Truck forum -
If the DSO code is two numbers, they refer to the District Sales (Zone) Office, where the original selling dealer ordered the vehicle from.
Prolly 90% + of all trucks assembled, have a two digit DSO.
6 numbered DSO: The first two numbers refer to the District Sales Office.
The next 4 numbers are the: Domestic Special Order number. In other words, the original purchaser ordered something that was not an RPO = Regular Production Order.
This can be special paint, special body, etc etc etc...the list is endless.
Why a 6 numbered DSO code? If the vehicle comes in for service, and the parts are not listed in the parts catalog for whatever is special, the dealer contacts FoMoCo, gives them the 6 digit DSO.
Ford looks up the order number, tells the dealer what the parts are, and where to get them if not Ford parts...or gives the dealer the specifics of the paint.
The DSO system was initiated in 1948 for trucks (1949 for passenger cars) and is still in use today.