Recently, I purchased an older restoration 155B Briggs Town Sedan (late November, '29) and had not thought about checking to see how close the front doors matched my late August 68A. The exterior dimensions of the two sets of front doors do match. The hinges appear to be on the same centers. The 68A does have wooden cross members internally to which the window regulators and door latch assemblies are attached. Internally, the 68A doors appear the same as my 60B. I suspect that Marco is correct and the doors could be the same as the 60 series. Since my new to me 155B has upholstery on the inside of the doors, I have no idea as to the door internal construction. It would not surprise me to find that the 60 series doors are the same as the Briggs 155B.
The door stamping above the top hinge is different between the 68A and the Briggs Fordors. I can't determine if it has been "restamped" on the Cabriolet or simply a different stamping. It certainly is an easy problem to overcome if you attempt to convert from one to another. Gar Williams