Here is what I have, and of course it is the usual 110 story where dates are only approximate. Production of the 5th Version, Variation 2 (inverted "Buck/110/USA")started in late 1972 or early 1973. The frames of Variation 2 were slightly smaller than those of 5th Version, Variation 1 (inverted "Buck/USA"). This was also true for Variation 3 ("Buck/110/USA") which started in late 1973 or early 1974. Early in 1975, Buck switched to sintered brass frames, which were much larger. The first of the sintered brass knives were stamped "Buck/110/USA". Late in 1975, probably early 1976, the blade, lockbar and spring were modified with the resulting "Buck/*110*/USA".
Bert