Close... Murray and Briggs did build fordor sedans for ford for 28-31. The bodies were almost identical between the makers with few exceptions for the cages around the body nuts and minor things like that. The biggest and most visible difference is the top of the arches for the windows on the doors. One had an arch, the other was flat across the top.
The doors are about the only thing that's not interchangeable between the two. In April 31', the fordor dropped the visor, and had a slightly slanted front windshield and A pillars as you mentioned. This was just a design change with Murray and Briggs continuing to make this body style.
The slant window fordors also have the arched window for both makers, making it harder to tell who made the body. The slant windshield fordors are more desirable since there is considerably less wood. These cars have a metal structure, where the 28- early 31's had a wood substructure.
For everything you want to know about Model A's: http://scootermcrad.blogspot.com/2009/03/everything-you-wanted-to-know-about.html