I bought a set of used S40 siderails.  The helemt lock was still on the siderail.  Of course, my key didn't operate the lock, so I took it off and planning to put the lock from the Savage siderails on when I swap them.  

I decide to take the lock apart.  I removed the small screws on the back and removed the cylinder.  It turns out there are 4 small plates that slide up and down in the cylinder. The key passes through the plates and moves them into position so the cylinder can turn.  The wrong key doesn't set the position  correctly, so the plates stop the cylinder from turning.

So I decided to swap the plates around.  I was suprised to find that I could get all 4 plates to line up just right with my key.  

Makes me wonder if they use the same 4 plates with all of the keys, so that any cylinder could be made to work with key.