During the period of 1965-1970, Case used the tang stamp CASE XX over USA, the so called "no dots" stamp, on all pocket knives. Starting in 1970, a row of ten dots was added under the USA. A dot would be removed each year, so 1971 stamped knives would have nine dots, etc.
A quirk of the P172 is that it does not follow the Case date stamp system exactly, because the pattern number is on the front tang with HAND MADE IN USA on the back tang.
Case introduced the P172 in 1969. The first ones made will have CASE XX over P172 on the front tang and HAND MADE IN USA on the back tang.
Starting in 1970, while other Case pocket knives had the ten dots added, Case forgot to add them to the P172 stamps, so a P172 made in 1970 will be marked identically to one made in 1969.
Starting in 1971, the row of dots was added to the P172 stamp, but on the back tang below the HAND MADE IN USA.
You have to be careful in determining the year of a P172 since the large bolster tends to cover the dots. Sometimes you will see a two dot 1978 or a one dot 1979 where the dot is hidden, so people think this is a USA "no dots" stamped knife. The way to tell the difference is that the later 70's P172's will have no oval arounde CASE on the shield, whereas the older ones will have the oval.
