Thanks, deport.
It looks like the bump up for Vietnam began in 1965 and ended about 1970. The nation was taking about 100,000 per year just for normal fill. It bumped up another 100,000 in 1965.
On to figuring out how they decided who got the lucky letters in 65-67 and 68-73.
The years prior to 1970 were based upon the system of usually the oldest available body without a deferment on the local draft board list got the call.... This is during the time of deferments, etc. Beginning in 1970 the lottery system was in effect..... The first lottery was held on Dec. 1, 1969 for those to be drafted in 1970 and included the birth years 1944 to 1950, if I remember correctly. There after the next birth year 1951's lottery was held for drafting in 1971... and so for each year up to the draft ended in 1973.......
The results for the four lottery drawings can be viewed here;
http://www.sss.gov/lotter1.htm