The unit mentioned on his his discharge papers is not necessarily a unit he served with while overseas. It probably is, but is not ironclad. Many units were broken up in Europe and the men rolled into other units for transfer home. Also he could have had enough points to go home and was part of unit slated to stay in theater, so he could have transfered to a unit that was headed home. Unfortunately, the last unit he was a part of shows up on his discharge.
Anyway, I cannot find a 451st Engineer Detachment Company in my books. Owen, you are correct, it was a corps asset, being part of VI as indicated on this page:
Order of Battle • VI Corps This is the only page I could find on them, must be the one you were looking at.