I think this is what you are looking for. The two line inverted ran from 1968 into 1972, and the three line inverted was in 1972. Someone else will have to give you information about why the stamp was inverted. All I can say is that the inverted stamp was what was used on all of the earlier knives, both one line and two line. A better question might be, "Why did the stamp change from being inverted?"
Bert