Gizmo688,
Back to the original topic, I think that knob tooth is one of the more severely worn I've seen. They are specified to be 0.018"-0.023" tall when new, with the sides sloping up at 35° and with the tip radius no greater than 0.003". They are case hardened "file hard" according to spec. I've used diamond hones to slightly sharpen dull ones very carefully in the past, but the problem is that the case hardening is only 0.004"-0.008" thick, and once you wear past the case hardened portion, then the receiver, which is also case hardened, files the rest of tooth off very quickly. Getting one of the discs with new serrations won't help because the tooth is slipping over tip of the serrations now. You have only one tooth and many serrations in the receiver, each of which not only has a deeper case, but which share the wear among one another where the single tooth on the knob sees it all.
Oh, one more think occurs to me to mention. The M14's sights are the same. You can get the knob for an M14 or an M1A and it will work. Fulton Armory has them in both their M14 and Garand parts lists on
sale for $44.96 at the moment.