Rear uprights

[alga]

I would hazard a guess that it's this way to save the effort on cutting. RU7 is round, because it goes into the tube, and the hole is round , because the round UJ has to pass through it, but the outside shape of RU8 is a nice envelope of the 4 threaded bearing mounting holes made out of 6 straight lines so it's easier to make. Also, it allows you to weld RU8 straight to RU7 in the 4 places where RU7 is not covered.