I agree with Dean, that it is very unlikely Parker Brothers work. Had it been, and there was a logical reason, it would have been more common. The reason was likely to chamber and shoot cartridges which had a thick rim, but altering the gun would have made for an unsafe headspacing if one was to go back to thin rims. What makes me go hmm, is why did they rebate the standing breech, and not simply deepen the rim groove in the barrels? The breech face would then still be perpendicular to the bore, and not a calculated machining angle. The reason the breech face is not a 90 degree perpendicular angle is simple, when you think about it. If it were 90 degrees, the bottom edge of the barrels would strike the breech face in closing.
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