The Collaborative International Dictionary of English v.0.48:

Countergage \Coun"ter*gage`\ (-g[=a]j`), n. (Carp.)
   An adjustable gage, with double points for transferring
   measurements from one timber to another, as the breadth of a
   mortise to the place where the tenon is to be made. --Knight.
   [1913 Webster]