Guess as to the "Round Notch"..
Hi Patrick,
I'll bet that notch will match the OD (outside Diameter) of the pin from the point it protrudes from the breechface to the tip of the firing pin. If it is "flattened" (and the guage won't fit over it - it will decrease the Overall length of the pin and its protrusion length (and be less then the 1.1 mm length on the guage). If the pin is narrower then that OD - my bet is that the pin would run the risk of piercing the primer casing when it strikes...
It's only a SWAG (Silly Wild A$$ed Guess) and probably not worth Frank's $0.02
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