It appears to be Merkel's own sidelock system, from what I have seen and from what I've been told by those in the know. I find Merkel actions on many of the pre war Belgian o/u doubles. The gun is still tight as a drum and regulates perfectly to the S&B 285gr loading. It doesn't shoot quite as well with Norma or RWS, but both shoot close enough to be perfectly adaquate. I use the BBF barrel set as much as the 9,3 set, as I like to take grouse and chucker while big game hunting. The bores are perfect, and the wood is as tight to the metal as the day it was first delivered. It has some of hunting dings, many gotten by me in Africa, and in NW Montana, hunting the clear cuts for deer and grouse.