I got some tools yesterday from an old toolmakers box my local scrap man had put to one side for me , amongst them was a guage with four indents milled out 1/2" dia leaving 0.025" 0.050" 0.075" anf 0.100" and it's 0.125 thick, there was a handle but that went into the skip and only realised today what that they went together , it's got to be for checking grooves but matches those sizes .