Need help with with (1) code on 68 RR fender tag

I've found that the pre 1969 / 70 Fender tags are a little harder to decode (at least for me) but I've been able to successfully decode most of my 68 RR fender tag with the exception of one code.

On the 3rd line from the bottom (CAPITAL LETTERS) I have the number "4" directly under the "C":

Example:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
4

Does anyone know what option this is for ?

Thanks in advance for the education !

That is #624: HD suspension with sway bar

Thank You !