"O" for Osamu ?

Upon stripping I found a small "O" or circle painted on the inner crankcases behind the sprocket. I have no way to be sure, but this mark may have matched the "O" or circle seen on the outer clutch cover, as clearly visible on Osamu Masuko's YD-A at Asama.

Other early Yamaha colleagues believe these marks may be associated with individual mechanics, but so far I have not found out for sure exactly why these symbols were used. .