We have covered this before, but here goes again. I will stay specific with the 41, though the 46, 47, and 48 are basically the same. The 41 business coupe can be identified by a "short door" which is the same door as on the 4 door sedan. The seat is not split: it is one piece bottom and back, but the back is hinged at the top so it can be raised up (has a latch to keep it up much like the trunk latch) and allow access to the big space between trunk and seat. There is a package tray installed behind the seat and was a great place for a 40's businessman to carry samples, catalogs etc. The quarter windows were fixed-could not be opened or rolled down. The rear deck was longer and provided the best body lines of all the 41s.
The 41 coupe has the "long door" which is the same door as on the 41 2 door sedan and convertible coupe. The front seat had a one piece bottom and a two piece back that hinged at the bottom to allow easier access to the rear seat which was a small bench seat in most models. There was optional two small side facing seats and thus coined the term Opera Coupe that us old timers have heard many times. The rear quarter windows could be unlatched and swung out much like the old vent windows. Some later models could be rolled up and down. The rear deck was shorter producing more stubby body lines.
If you need a left frame rail, look for any 41. The rail dimensions are going to be the same. Even though there are some minor differences as to fender and running board mounting patterns, the frames rails were stamped with holes for all of the configurations. Dimensionally, the rails are the same up thru 48, but the hole patterns change slightly but don't turn one down because of that!!! Forget about 40 and older. Totally different.
By the way, I had all the literature on this stuff at one time and still have access to it when ever I want to make the 1200 mile trip (one way).