I now believe the "Logan Guards" were the 46th Pennsylvania which lost a flag or two at the battle of Chancellorsville. This is the only 12th Corps flag I have found so far. Knipe's Brigade, William's Division, Slocum's 12th Corps.
The was listed on the returned Union Colors from the War Department. It was also on the list of 58 Union flags identified and returned. I post that on here last summer.