Raimey, from your first photo, I would guess that while ATM produced guns, they didn't build Merkels from the bottom up. "Licences Merkel frères" . . . licensed to sell or to make? I'd guess the former. Although Dave's gun with the ATM logo on the bottom of the receiver . . . maybe they did some finishing on guns they received from Merkel?

It will be interesting to hear about the proofmarks on Dave's gun.