Lapel pin question

[Mike Serpa]

These men seem to be wearing the same lapel pin. They are Civil War Medal of Honor recipients. There are 17 more MOH recipients with this pin in post-war photos in the very same manner. Two are from from later wars. Any ideas what this pin is?

Congressional Medal of Honor Society photos

 

[lupaglupa]

The third pin looks a bit different to me

 

[major bill]

I am not sure what they are.