The Collaborative International Dictionary of English v.0.48:

Coag \Coag\, n.
   See Coak, a kind of tenon.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Coak \Coak\, n.
   1. (Carp.) A kind of tenon connecting the face of a scarfed
      timber with the face of another timber, or a dowel or pin
      of hard wood or iron uniting timbers. [Also spelt coag.]
      [1913 Webster]

   2. A metallic bushing or strengthening piece in the center of
      a wooden block sheave.
      [1913 Webster]