In general, I like the idea of using a letter as a graphic, just none of these are quite right. I like the use of a drop of oil(#43 with bit going through it), but look for a picture of an oil drill bit. take a look at this picture, this is a fairly accurate representation of a mud motor(our type of tool) attached to a drill bit. http://www.epmag.com/Production-Drilling/Ensure-consistent-directional-control_3698 not looking for exact image, but similar as you used a screw in 43.