Company A-21, 6th Regiment was either a training or staging unit. His unit during basic training or while awaiting assignment at some point during his service prior to going overseas. Fort Eustus was an Antiaircraft Artillery Replacement Training Center. Company A, 12th Antiaircraft Artillery Replacement Training Battalion was the unit he trained with at Eustus.
Normandy, Northern France, etc. are the campaign credits to which he's entitled. This will help you determine the number of bronze service star devices authorized for his European-African-Middle Eastern (EAME) Theater Medal but little, if anything else.
103d Antiaircraft Artillery (Automatic Weapons) Battalion was organized in February 1941; overseas in April 1942. Returned to the U. S. in November 1945 and was inactivated 1 December 1945. He may have joined the battalion as a replacement in March 1944 but most likely transferred into 103d just prior to its returning to the U. S. Only way to determine for certain is to access records.
As for his grave marker inscription. Very often the mortuary arranges for the marker. The next of kin provides a copy of the decedent's discharge and the mortuary lists the unit indicated on the discharge in the application for the marker. (If it turns out he did, in fact, serve with another unit overseas and you'd like to replace the marker, I'll show you how to handle that.)
If you haven't already, you can request his personnel record from NPRC St. Louis. It was probably destroyed in the fire there in 1973 but it's worth a try. They prefer that you submit requests using Standard Form 180. You can download the form from the NARA web page at < http://www.nara.gov/regional/mprsf180.htmlhttp://www.nara.gov/regional/mprsf180.html > (click on the address to go directly to that page). The mailing address is on the form.