Leanwolf,
In approximately 1968 (perhaps 1967 on some models), Buck added the "U.S.A." to the tang stamp. These knives are also referred to as two-liners (BUCK / U.S.A.). One liners, of course, were stamped "BUCK". Then in late 1971, Buck began to add model numbers to the stamping and these are called three liners. Some of us call the 1986 and later models with a year symbol four liners even though they are actually three with the added year symbol after the model #. Remember that these dates are almost always approximate as Buck would have used up existing inventory before starting the new variation.
Larry
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"He is no fool who gives what he cannot keep to gain that which he cannot lose".