How to tell MKI header from MKII header?

Looking for stock replacement headers... Marking includes: "EM-WUC 131E-L (or R) with Arvin Meritor below the numbers. Other headers have the same numbers but no 'Arvin Meritor'. And, some headers state: EM-WUC 131E-L 01 (or R 01), with no Arvin Meritor. Can anyone tell me which are MKII headers? I'm wondering if the headers marked '01' might be a later build? Or, maybe the headers not stamped Arvin Meritor might be later build since Arvin Meritor changed its name to Meritor in about 2008? Help.