SU Needles-identification

["Scott Gardner" <gardner@lwcomm.com>]

Hey all,
        Just tore down these SU HIF4 carbs that the PO dropped off.  The 
metering needles are marked "ABD".  My Vicky Brit catalog lists that 
as the correct needle for the '73-'74 models. (Mine's a '72).  
Evidently, the metering needles in my carbs should be AAU.  What's 
the difference, and what's the usual reason for switching needles?  
Is there a noticeable difference between these two needles?

Thanks,
Scott