A union of structures; one in which you can change the number of members and the other fixing the size at 256 bytes and fixing location of the error number. To something like:

typedef union h
{
    struct
    {
        U8    SDser[8]  ; // serial number
        U8    SDaddr    ; // address assigned to sign
        U8    SDdips    ; // dipswitch setting
        U8    SDtype    ; // type of sign
        U8    SDwidt    ; // width
        U8    SDhgth    ; // height
    } sd;
    struct
    {
        U8    SDfill[255];
        // put this one last
        U8    SDerno    ; // here store error number if FRAM match/update failed.
    } er;
} SIGN_DESC;