The third pin serves a couple of purposes.

  • It keeps the blade from swinging all the way through the scales
  • It keeps a workable dynamic between the blade and the scales goin' on when the blade has quite a bit of taper on the tang. It is mainly to prevent cracking the scales on those heavier blades when they are opened and closed. The third pin serves as a spacer to match the width of the blade at the point on the side of the scales where they match up together.

If the pin has been placed in the correct position, it will serve as a rest for the blade. You can see what I am talking about if you go to Making Scales 106 on my razor blog...

http://razortips.blogspot.com/

The pins I've seen are always smaller than the pivot pin and rarely have any washer or collar. Is it safe to peen a pin like this?
It is a customary way to attach pins to materials like ivory and mother of pearl in this manner. You just have to know what you are doing. The trick is to mushroom the head and not expand the pin as you are peening it. This is accomplished with light taps. Nothing harder than with the same force you would hit the end of your finger with the hammer.

Don't take offense, but judging from the way you asked the question, I would suggest you practice with some other materials first before you start with the ivory.