I heard about this from tedaut, details here:
Running 2K+ cocker off unregged 12g - springing recommendations? - Custom Cockers Forum
I'll quote the relevant bits here, but there is more info on the original thread
"AL Cable Stop
5/32" Diameter
This goes in the valve chamber before you put in the valve spring and further reduces the volume. It also acts to put some preload on the valve spring."
I will be transferring the guts from this gun to another body and can take pics if you want.
Just to bring this back OT - this is something that is possible with cockers, what about Palmers?
w0se