57c barrel hook zinc round have a 'pin' projecting on one side. This was bought as a spare in December 1979, and this pin is the same hand (upper with hook open to right) as both of those shown in OPM. The pin may be part of a master used to spark arrest a mould, - it looks as though it may have held the hook within the barrel - or it may just be an ejector pin. I didn't have a large enough sample to say whether there were left handed versions, but I now have one...