Charlie, here is a condenced unit history with the link to the web site and another link to a book that has an additional discription of the unit insignia. The last link takes you to an image of the insignia on line. NOTE; that this insignia was not approved until 1957. If there was another insgnia specifically for this battalion, I haven't found a reference for it.
400th MP BN Constituted 19 NOV 1944 in the Army of the United States as 400th Military Police Battalion (less Companies A and B). Activated 20 NOV 1944 in France with new companies except for company A redesignated from 181st Military Police Company (activated 20 JUN 1943 in Algeria) and Company B redesignated from 185th Military Police Company (activated 26 JUL 1943 in Algeria). Inactivated 9 APR 1946 at Camp Kilmer, NJ. Allotted to the Organized Reserves, assigned to Second Army and activated 1 MAY 1947 at Washington, DC. (Organized Reserves redesignated 25 MAR 1948 as Organized Reserve Corps; redesignated 9 JUL 1952 as Army Reserve). Inactivated 25 MAY 1959 at Washington. Activated 16 SEP 1989 at Fort George G. Meade, MD. CAMPAIGN STREAMERS WWII Rome-Arno Rhineland COAT OF ARMS SHIELD: Per bend vert and/or two cocks counterchanged. CREST: That for the regiments and separate battalions of the Army Reserve: On a wreath of the colors (or and vert) the Lexington Minute Man proper. The statue of the Minute Man, Captain John Parker (H.H. Kitson, sculptor), stands on the common in Lexington, MA. MOTTO: Watchful Always. The colors green and yellow are for the Military Police Corps. The cock stands symbolically for vigilance, warning, and watchfulness. The two cocks on the shield represent the two campaigns of the unit in Italy during World War II. DISTINCTIVE INSIGNIA The insignia is the shield and motto of the coat of arms. The sample of the insignia depicted (http://www.wood.army.mil/usamps/history/MPunitHistories/Battalions/400thMPBn.jpg) was approved for wear on 10 MAY 1957.