3/8" 90° countersink, double end, 6 flutes.
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 90° 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 chatterless grind keeps the 6 teeth in contact with the work rather than skipping across it, which is what stops the tool from cutting a lobed, faceted seat.
90° is the standard included angle for metric flat-head screws, and the common choice for chamfering a hole before tapping.
