Note that guns built like this follow the original 1881 patent design. On these the breech piece is bored through on a slight taper & the tubes inserted from the rear. This is the reason for the step at the end of the block. On some later models the block was threaded part way through from the front & the tubes had a section turned & threaded to screw in, but extended only about half way through the block. These do not have the step at end of block but a smooth transition as in modern mono-block guns. Somes guns constructed as the one in Pete's ref are marked "Original Diana" while one I have of the screw-in type is marked "Modified Diana". I am however unsure if this is consistent with the construction method, or is a grade marking. The Modified Diana guns I have seen do not seem to be as high a grade as the Originl Diana.
