Gentlemen--During my years of collecting and doing research for the new Smith book,I've come accross several so called 5-pin guns all on regular weight frames. Whether they were factory or not I don't know but what I do know is that the extra 2-pins line up perfectly with the upper front and upper back blind bridle screws found on at least 99% of the guns. The 3-lower pins are of course the tumbler axle pin,sear axle pin and main spring locating pin. Just something to think about. Hope this helps a little. If you come up with something more definitive, keep me posted. Good Luck !!