Are you talking about the little retaining pin that holds in the axis pin? #13 in this picture? I don't have one in my hand yet (my No.5 is missing the sight), but I understand that it's a straight pin. On another forum, I've seen guys mention that they've used several things to replace that retaining pin, like little nails or brads that have been clipped to the right length.
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I child-proofed my house, but they still keep getting in!