Yes, that is the basic set-up. It's also used on swivel breech designs. This can be seen in the Simonin prints as well. I do think the gun in question is set up a little differently, however. The plate coverage to the rear of the cock is significantly smaller in the HH lock. By looking at the visible screw holes, I don't believe the entire mainspring can be covered by the plate. This is why I referenced the Heinrich Raab print. I will post a scan of this. As I mentioned I believe the spring extends beyond the plate to the towards the butplate. This is the design shown by Raab. With such a set-up an undercut is created in the stock and the protruding mainspring hooks underneath it. If you look a the visible screws, it seems the one directly behind the cock must be the sear pivot screw, the next seems to be the mainspring anchor screw and the one at the tip would likely be the sear spring screw. I suspect a straight sear spring was used rather than a bent one. Considering the placement of the screws, the size of the plate and the size of a mainspring, I question it can be covered by the plate. Now I could be all wrong, but this is how it seems to me. Now all we need is someone to buy it and show us what it really is!