Got this off the CR list quite some time ago...

1921-1936 - The first character is a letter and has a sequence of a b c d e h k l n p s t v w x z
1937 - 1948 - The second character is a letter and has a sequence of a b c d e h k l n p s t
1949 - 1960 - The last charctewr is a letter and has a sequence of a b c d e h k l n p s t

My frame has a second series coding with a D for 1940.

And as for Operator... you know that if Sheldon was here he would have given you a polite swat for discounting his much beloved Twenty's as being impractical and may have told you about his wife dropping roadies on while riding a Twenty, with a baby seat !

If you said this in the fixed forum they'd also be calling you on your badly mistaken information.

I find my Twenty's to be ideal for urban assaults and even plan on pulling off a century (metrics are too easy) on my fixed Twenty.