The first five #'s are the model number 1st symbol is a 4 - indicating Buick Motor Division 2nd - 5th indicate the series and body style (2dr/4dr/sedan/coupe/HT/etc.) 6th number represents the year '1966' 7th number is the plant code: B=Baltimore C=Southgate, California D=Doraville, GA. H=Flint, MI. I=Oshawa, Ontario, Canada K=Leeds, MO. V=Bloomfield, MI. X=Fairfax, KS. Y=Wilmington, DE Z=Freemont, OH. The last six numbers represent the sequential order of production at each plant. Hope that helps you out

Sources: Standard Catalog of Buick 1903-2000