OK, back again. Trying again to see if I've figured it out yet.
12B = 2nd week December
Vin date chart on CRG is 9N551862 = November and 9N569987 = December
If those Vins are begining numbers for November and December than my Vin falls into roughly 3rd week November
My Vin 9N561885
Tank date is 18 69 which = 41/2 months from what I believe production model starting about August 1968
Then if my calculations are correct Aug, Sept, Oct, Nov, 1/2 Dec would fall roughly into my 12B build date
Looks good to me! Am I getting close to having this figured out? Thanks guys for trying to push me in right direction. Still a little slow but hope I've figured it out.