has to do with the radiator mounting pads, one is depressed where the radaitor ears would sit while one is flat... Sorry i cant remember for sure which si what... I bought one for my 29 AA (different than cars) from Berts in Denver for an excelent price and in near perfect shape.
This is a 30-31, you can see the radiator pad is below the frame rail.
This will work as you can shim the radiator up to fit. The 28-29 will be
even with the top of the frame rail.