They ALL have the same spring and piston assembly.
The gamo 1000 series 220/440/680/hunters, 1000 Shadow .177/.22, Silver Shadow .177/.22, Shadow Hunter .177/.22, CFX .177/.22 and all the rest are all the same power plant. they have minor differences in the top hat but I have only seen two types. one is solid and one has a 1/4 inch nylon spacer under it.
I have Rifles with both types and can find no difference in the crony output. I even hve a new Nitro 17 and it has the same spring but a smaller top hat, but it still puts out 795's on the crony with 7.9 CPL's
Gamo's like many other manufactures just change Cosmetic options to make different rifle styles.
I own and shoot 7 Gamo rifles and have every one apart and a few other gamo's I have tunned for others.
2 Shadow Hunters, 2 CFX's .177/.22, 1 220 Hunter, 1 Nitro 17,
1 Shadow Supreme .22, 1 Shadow 1000