ZYX SB and ZYX GB. Silver Base (SB) adds about 4 grams to the cartridge and Gold Base (GB) adds about 6 grams weight to the cartridge.
The nature of the sound becomes somewhat different when you use ZYX SB or ZYX GB. Overall, I think SB provides the better value and we provide it as a gift (i.e. product feature) to our customers with the purchase of a ZYX Airy 3 or above. We think all ZYX cartridges benefit from the added weight and that the added weight should be included as a feature, hence our offer of this option as a gift (i.e. feature) for airy 3 and above, or at minimal price for ZYX R100 fuji and below to our customers.