Perhaps so. This represents to me the cheaper, quicker conversion method, as most of the Euro conversions I have seen used a patent breech method. More expensive and took longer, and far more elegant. I would expect a gun of this quality to have had that procedure, than the simple drum. Bedford makers made lock plates with blast protection flanges and you see it elsewhere occasionally. This isn't a Bedford hammer of course and you're right on about the fact that it isn't typically American. To my eye, however, the hammer looks Yankee as many of the hammers over there have swept lines, somewhat dolphin like: many almost sculptured. This job just isn't consistent with the rest of the gun, to my way of thinking.
Dick