G'day Alan,
The primer bulbs to fit the carby caps have never been available as a separate part, as far as I know. As the plastic of the cap is hot crimped over the bulb flange during manufacture, it's not designed to be replaced other than as a complete assembly.
As far as distinguishing OEM from aftermarket, the genuine Victa caps have the Victa part number, CR03501A, in a moulded panel above the 'level line'. The aftermarket caps I've seen here don't have this feature, for what that's worth..
