You must be talking about 47's and early 48's? Those trucks were not considered F-series trucks. Sometime in 48 ( Mid year ) is when the F-series came out. THat series went from 48-52. THose trucks had the 3 ribs in the back wall. Some of the larger F-series didn't have those ribs. My 50 F-1 has a cab from a 49 F-6 and it does not have the ribs and a f-6 cab from a 50 doesn't have the ribs either. My original cab from the F-1 did have them. Any other F-1 thru F-3 that I've seen had them. AS for the earlier trucks, I can't vouch for those.