There are two slightly different parts diagrams for the Diana Mod 6 airpistol that I saw on Danny's VAG web site.
The two diagrams show the earlier and later versions of the Mod 6.
The earlier version shows 3 parts that make up the cap, cog-wheel and shaft, numbered as 30/1, 30/2 and 30/3 respectively.
I have never handled this type of 3 part assembly as the Model 6 and the Model 72 actions I have dismantled were late models with just two parts, a cog-wheel and its retaining cap.
Looking at the diagram of the 3 part assembly, I would think that the shaft is a press fit into the cap. Whatever you do, make sure you protect the threads and cap itself before taking the shaft out.