The unit on his grave marker may well have come from his discharge, which means it may or may not be the unit he served with overseas (and probably isn't). See whether your Grandmother (or someone) has a copy of his discharge papers. This will give you a starting point.
(If you know where he entered service from, check with the county/parish clerk there. If that jurisdiction paid a bonus for WWII service, he'd have been required to provide a copy of his discharge to prove eligibility. Likewise, any jurisdiction(s) where he's lived since the late 1960's may have a copy on file.)