Some of the cases are closed off on the countershaft where the factory brake would mount. If that were the case, you would need a countershaft that does not extend out the side of the case.
There are some that use bearings vs. bushings for the output and/or countershaft. These are not as common.
For the most part, the big differences were all in the internals. Gear sets and number of gears.
If you have a case that is open on the ends for both shafts, and uses bushings, it is interchangeable with any other case that is open on both shaft ends. If it is closed off on the countershaft, they are also interchangeable provided you have a countershaft that does not have a factory brake mounting provision. Or, chuck it in a lathe and cut the brake mounting splines off.
Do not use the gray steel bushings. Use the bronze bushings.