You can swap either way as long as you account for the pinout change. There are some modern keyboard adapters for both A and B machines available now as well.
I was a bit incorrect. The 26-1138 is just the WD controller for the Model 4 that allowed it to use the 25-1041 as its primary drive. I should have remembered as I have one somewhere. The 25-1041 could be used by any MFM controller including the 26-1138 or the Tandy 1000 controllers.
The other one is a 26-1138 primary drive intended for the Model 4. It has the 50 pin connector on the back and a full WD1010 controller board. The 25-1041 is a dumb secondary in this configuration.
You guys are too funny. I only have half of the 1984 production run. :)
The answer is 95%+ of them have been scrapped. They are too big to just chuck in the closet.