J. W. Nichols, watchman at the postoffice building, Omaha, belonged to Company K, 150^th Pennsylvania infantry, that was detailed in August, 1862, as the personal bodyguard for President Lincoln, and continued on that duty at the White house until June, 1865.  Mr. Nichols says: 

“I saw Mr. Lincoln almost daily from August 1862, until his death, the early morning of April 15, 1865, and was afterwards one of the bodyguards for President Andrew Johnson until our final discharge from the service in June, 1865.  We were quartered in temporary barracks just south of the Treasury department building in White house grounds.  It was our duty to escort Mr. Lincoln to and from his summer home at the Soldiers’ Home during the summers.  “We all came to know him personally and he knew most of us by name.  Tad Lincoln was at our quarters most of the time.” 

“Tad was a most lovable young fellow, and was the favorite son of the president, and for this reason, I think Mr. Lincoln always had a very kindly feeling for Company K.  One thing that always rather amused me regarding Mr. Lincoln was the way in which he would respond to a salute.  He always wore and old-fashioned plug hat, and in returning a salute he would awkwardly swing his long right arm around back over his shoulder and grasp his hat by the top instead.

From a  newspaper article