Slinky 75 - I can confirm your Grandfather was in HQ Co., 824th Engineer Aviation Battalion. The reason I state so is that I paid a researcher, Golden Arrow Military Research https://www.goldenarrowresearch.com/ to attempt to find my Dad's records. Geoff Gentilini, who did an excellent job finding and summarizing what he could find in Company Morning Reports, Company Pay Rosters, and a most crucial document for you, sent me a .pdf file containing the Order which sent our ancestors on to a Replacement Depot from Iceland to be plus-ups to other EABs in the event they took combat losses.
Your Grandfather is listed as Tec 4 Hamilton C Snell 3301266?4 Hq Co on the document I have for "B" Co ( all others are lined out, so your Grandfather is listed but crossed out, making out his serial # tough ).
I'd suggest you get in touch with Geoff at Golden Arrow. He did an amazing job filling in more of some of the things I knew, and finding things I did not know about my Dad's service, and what I paid for it was well worth it to me.
I'll try to print out that Order and attach it in a next post, hope I am successful, and I hope you see this many-years-later reply.
My younger daughter took me to Iceland and Normandy in June 2017 to retrace my Dad's footsteps. It was an emotional event, to say the least.