1stLt is the Air Force shorthand. Army is 1LT. However, this memo uses 1st.Lt. (with periods, which is not used). Same for the appearance of "Lt. Colonel" (Air Force shorthand is Lt Col, Army shorthand is LTC...Killian has signed other documents available as Lt Col - no period and shorthand Col).
My husband was in the AirForce and he said 1st was never used to his knowledge. He believed it should be 1Lt. or 1/Lt. In most of the documents it seems to be 1Lt.
Thanks, btw. I appreciate your input. :)