lapsub.
From a book on proof I have......

Crown/U Supplementary mark signifying inspection or view after proof.

Crown/E Supplementary mark applied to express rifles.

Crown/G Supplementary mark for barrels firing solid projectiles.

Yeah, those clamshell actions look very strong.
JP Sauer built doubles on that type of action.

Could "bemesu" have anything to do with the type of steel used in the barrels?

I'm sure someone here will know.