Hi , I believe the letters CAC stand for Coastal Artillery Corps, which would be borne out by the battery designation on his tombstone. Here's a possible place to start, http://www.getnet.net/~1stbooks/marin.htm
Hi Gus This first link designates the 66th as an anti-aircraft regiment that became the 135th AA Group during WWII.
Here is another link that gives a load more information on the CAC, http://www.cdsg.org/CACunits.htm, the first heading takes you to a PDF file that gives a fairly detailed history of the development of this branch of the artillery and its units and their duty stations. Pretty detailed stuff as the artillery transitioned from horses to motors & airplane defence into the 20th Century.
I know very little regarding the US CAC but understand that it was one of the most cutting edge technology-wise branches of the Army in those years.
This was a young officer who went through the Plattsburgh, NY officer training (the beginning of the 90 day wonder I think) and then shipped out to England. So I know his unit went overseas and eventually to France.
The link in Larmo's post is broken - can you re-post it?
Thanks,
Eric