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<comboBox 
 id="myComboBox" 
 label="My ComboBox:"
 getItemID="CallbackCBGetItemID" 
 imageMso="PictureEffectsGlowGallery" 
 onChange="MyComboBoxCallbackOnChange" 
 sizeString= "WWWWWWWWWWWWW">
</comboBox>

Sub CallbackCBGetItemID(control As IRibbonControl, _
                        index As Integer, _
                        ByRef itemID)
    itemID = Format(index, "00") & "a"
End Sub