The flat rib was used almost exclusively on number 2 and 1 1/2 frame guns in this time period. PB probably kept a small inventory of frames ready for use. Ed's gun is a 3 frame and probably the only one with a flat rib. Instead of making a frame just for his gun, they used the dimple. The flat rib frame was used latter on the vent rib guns as well. My 2 cents.
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