The CB750 uses a 2.15 rim whereas the 500 just got a 185 rim, so someone clearly has reshod the existing hub with a wider rim, most likely to use larger section tyres, I did exactly the same but used Lester wheels to get the same result. Probably thicker spokes because the wider rim needed more strength from them and the holes in the rim are larger. That's a good find, well worth keeping as it is and matching it with a 2.15 front rim would make it even better. You can use 4.10 front tyres then and 4.25/85 rears as I did back in the day.
Standard bearings were orange faced like those, so those are original.