Hci Paul,
I think S/No prefix "GJ" is for a Mk5 Airsporter, circa 1971-74.
Best pellets would probably be something like the old Eley Wasps in 5.6mm, the ones with light blue lid - not to be confused with the modern Wasp pellets (often touted on the bay, etc,)...which are rubbish in comparison If it doesn't say "ELEY WASP" on the tin then don't bother with them. You may also want to try RWS Superdomes, which are a good allround pellet for the older guns too.
As for value? no idea...sorry.
HTH