3/8" 60° countersink, double end, single flute.
A countersink cuts the tapered seat that lets a flat-head screw finish flush with the surface. This one cuts 3/8" diameter at a 60° included angle, 2-1/2" overall.
It is a double-end tool: there is a cutting end at each end of the body. When the first end loses its edge you turn the tool around and carry on with the second, so one tool lasts twice as long before it has to be reground or replaced.
The single-flute chatterless grind cuts with one edge through a cross hole. One edge cannot bounce between teeth, which is what stops the tool from cutting a lobed, faceted seat — and it clears the chip out of the hole instead of packing it.
A 60° included angle is the usual choice for deburring and for centring work.
