[Mr70]

That's true up to a point.
Originally the tips sold OTC w/o a part # stamped in them.Sometime around the late 1970's,early 80's they started to stamp them in.Exactly when I don't know,but I've seen a few sets NOS in the box that did not have a number stamped into them,and I have a set here in the box dated 1976 that doesn't as well.
GM later issued the license for these to be reproduced,and they released them with the part #'s stamped in the bottom,but some have been found on top as well.