That’s what the B210 series and B310 series manuals say alright! Had this tranny novice perplexed too. Thought I had your same problem once, but after I broke the transmission apart, I found the Rear extension bushing to be in good shape and it turned out to be the Rear Extension Dust cover and oil seal assy that was loose.

SKL Rear Extension Dust Cover and Seal Assy P/N 11619 for FS5W60A

I made a note to check this when I opened up four other Datsun 60 series 4 and 5 speeds- all had acceptable bushings. I was worried because I have had not seen entire transmission rear extension cases for offered up for sale. And after all this is said, you may indeed have a bad bushing, this is just my experience. If it is your bushing, a specialty transmission shop should be able to adapt one for you.