I haven't heard any issues with either of the axle housings. Either should work fine. They switched in 1950 I believe, or late 1949. As for the yellow, yes, it is Buff primer. That is what Deere used back in the day and is still commonly used as a primer for restorations.

The square tube was introduced at sn 248257, I doubt its any more structual than the round. I have heard some of late B's may have had casting problems from the factory causing crankcases to crack around the brake holes but it may be from abuse or an isolated issue. I believe the square tube was more of a safety change to prevent injury from slipping and falling, along with a new mounting system for tractors going to states with fender laws. A 1952 should also have a waterpump as where the earlier models have a thermosiphon system. The cast centers for the rear wheels also had a different look than the 1950 and earlier. 1952 was the last year of the B before John Deere went to the number series but both are pretty similar. If you have a prefrence on decals the 48 will have the JD decal centered on the hood where the 1950+ will have the decal near the grilles. As for the yellow "paint" it looks to be JD Buff primer sealer from an older respray. It is sold at JD dealers as a sealer to go over the bare metal before the enamel is sprayed (there a video on my channel of it on a B if you would like to see how it compares to yours). If you are going for originality the parts should stay with each tractor and not intermix. If it were me I would choose the nicer of the two and fix that one, or the one that has the most sentimental value to you. I am personally not a fan of robbing parts off of one machine to fix another because parts are plentiful for those old tractors. The 1950 B on my channel was bought to swap parts with a 49 B I am finishing up now but I didn't want to take a complete tractor and make it just a parts tractor so I fixed them both.

I personally like the round axle better... just for looks. I do know that those bright green chevys use a yellow primer before the green goes on to make it appear brighter I guess... so if deere did it I have no idea but it kinda could make sense I guess. Best of luck.

replaced the rad in my 1950 B in 2018 with a new one! could not find good one used .... Mine has the square rear axle and a wide front-end, newer models like the JD 50 even JD 520's kept the same style axle.. I think the squares have more weigth to them? not sure?.. as for witch one fixing up?.. both are worthy for fixing up!.. comes down to "your" hearts desire!..

What extremely little I know is I was told in a convo one time that the round axel was prone to cracking and the squared axel fixed that also allowed for different fenders to be mounted clam shell I think they call them not an expert though.

Sorry, I don't have any advice. But, I know rebuilding tractors are painstaking expensive. Be nice if you could redo both. Or sale one to help pay for fixing the other. These are highly sought after tractors. Best of luck with your journey.