The stereo version of Thank You Girl features harmonica overdubs in the middle eight (over the repeated "way that you do" line) and closing which are not present on the mono version. The mono version released in the US (on The Beatles' Second Album) features the harmonica overdub as it is a folddown of the stereo mix. However, the mono mix appearing on The U.S. Albums does not feature the harmonica overdub.
"We knew that if we wrote a song called, 'Thank You Girl', that a lot of the girls who wrote us fan letters would take it as a genuine 'thank you'. So a lot of our songs were directly addressed to the fans." - Paul
"'Thank You Girl' was one of our efforts at writing a single that didn't work. So it became a B-side or an album track." - John
"[The song was written by] Paul and me. This was just a silly song we knocked off." - John, Hit Parader
"[Thank You Girl is] a bit of a hack song, but all good practice." - Paul