1990 shorty....2 timing marks on the balancer. both look exactly the same. which one and what is the other one for?

i have a 90 astro shorty....just figured out that it has an egr valve on it that has a back pressure transducer attatched to it. took a while to figure out what the @#$$&% it was. if anyone knows what it is, can you just put a regular egr valve on it. cant seem to find one that looks like it at ...