Ok, I was really bored so I completely tore down my 4006 to bead blast some shiny holster wear spots on the frame. Here is what I discovered when putting everthing back together. There is a small difference between the 5904 and the 4006. The 5904 has a small copper tab or bracket riveted on the trigger transfer assembly (the thingy that goes from the trigger back to the sear part behind the magazine). The 4006 doesn't have one-but it does have a hole for one to be fitted. Is there one missing? Should I care? Will Smith & Wesson fix this?
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Pictures:

