If those valve covers indeed have the embossed (raised) decal, they are late '63, which means that the "O" couldn't possibly relate to chrome covers, as none were in production then. The same style stamping of the rectangular decal area began in mid '63 but as of '64, the decals were just flat with no raised embossing.
My information is correct, but in my opinion, those covers you showed look too good to be original. (46 years old??)
Valve covers were installed on the motors BEFORE they were painted. Decals were appplied on the engine dress line. How come they have paint under the screws? (not to mention there is no evidence of screws being attached AFTER paint)
Verne