Actually, the rear end housing was the same basic design, and the last 1850s, (serial number 199 340 and up) used the exact same bull gear as the 1950-T. But according to my parts book, the 1850 never used the axle housings like you are mentioning, part #163 631A. So that is interesting, especially with that 1850 having a 431 transmission code.

Once I looked up the axle housing in the 1950-T book, I knew which one you were talking about. I also know there were some bull gear troubles on the 1850s, and there are 4 different bull gear replacement packages listed in my parts book, that also included replacement axle housings. Maybe this 1850 was updated late enough that they used the 1950-T axles instead of the other one that came listed in the package. Your friend's 1850 has a serial number that falls into the range of ones that were updated, so this is my best guess.

Looks like you get the eagle eye award for noticing the difference!