The "narrow" pickup beds alone were known to Ford as 78A for both 28-29 and 30-31. Used with either the open cab or closed cab trucks.
The Closed Cab alone used in 1928-29 were 82A
The Closed Cab alone used in 30-31 were 82B
The Roadster pickup body alone in 28-29 were 76A
The Roadster pickup body alone in 30-31 were 76B
The "wide bed" pickup bed 31 mostly alone was 78B I'm not aware of any RPU bodies being used against a wide bed.
There were VERY MINOR differences between the 28-29 and 30-31 narrow 78A beds mostly in the riveting and the "impress" of the tool used to create it. Most, even owners, would not know the difference. And given the "delay" that assembly of a truck might involve, even assigning a specific rivet/pattern to a specific year/assembly is dubious. It is not usually examined/critiqued in judging.
Joe K