For regular closed cars, the frames are the same size between 28/19 and 30/31. However the slide arm bracket and hinge are different between the 28/29 and 30/31 model A's. The 30/31 have a two step angle cutout on the end of the wind deflector where the 28/29 have one angled cut ends. The slide arm brackets make the windshield not interchangable, however the brackets can be changed to make the windshields fit between the 28/29 and 30/31's. Brigg and Murray windshield I believe are different from the regular Ford made closed cars. Of course open cars have totally different windshields for closed cars as you probably already know.