Pluck, The only RGJS information I find is page 8 (revised 2011).
"Vehicles which were produced by Ford with only a cowl
and chassis (sub assembly) must have an appropriate body
provided by an outside supplier. This includes the Chassis
Seat model , Ford body style 75-A."
This statement does not reference a cab and chassis unit. But, it seems there is an unwritten rule that a cab and chassis must have an outside supplied cargo body to be judged or points are deducted.
There is a list of "A" chassis body types but no body type identification information. The E supplement (Heavy Commercial Vehicles) has eleven pages of AA body types identification.
I don't see any way for a judge to be able to judge a outside supplier body or even to know the difference between an era supplier or a home made body.
There must be some unwritten rules regarding this procedure since non-Ford supplied bodies are being judged at national meets.