I bought a 4/71 500K0 last year (VIN < 1000), with the early 'squared-off' top crankcase ( as pictured in the Service Manual). It was a US import and not used for many, many years. I completely stripped down the engine, which had never been modified, and the only thing I could find untoward was the needle roller bearing on one of the transmission shafts had a broken needle retainer cage, which I have never seen on any of the numerous CB350/250K twins I have owned, which essentially use the same bearing. If the bike had been used for much longer I am sure that bearing would have failed. I noticed on the later CB550 that the bearing in that location in the transmission was changed to a different type, fitted with its own little 'trochoidal' oil pump attached to it. So I would inspect that 500 bearing very carefully for damage on any early 500's. I think it was the one on the countershaft.