A few years ago I was looking at two SXS Merkels for sale. One was a box lock, one a side lock. We carefully broke them down, kept them separate. I was told the same guy sold them to the dealer. I was checking the serial numbers and someone [either the original seller or the dealer] had the forearms switched around. I asked the dealer to confirm this and he verified it. They were both a tight fit, but interchangable.