The Leif Ericson model did include a snazzy bug-eyed scoutship which fits snugly into the hangar. The canon text mentions that the Leif carries two scoutships, which must have been snug.
Here are some reference scans of the plastic model.
After the first Star Trek TV series ended, in 1975 Matt Jeffries was working for George Pal to work on a TV series based on THE WAR OF THE WORLDS. The good ship Pegasus needed a scoutship, and Matt Jeffries designed a good one. Sadly the TV show was never picked up.
Later in 1977, Gene Roddenberry tried to start a new series called Star Trek Phase II. Matt Jeffries returned for some re-designing. Matt was a pilot and an aviation artist, and was always annoyed at the box-like Shuttlecraft Galileo (box-like to keep construction costs low). His redesign was a much sexier shuttle. And since it was a Matt Jeffries design, it fits with the look and feel of the Leif. The following images are from Star Trek Phase II: The Lost Series by Judith and Garfield Reeves-Stevens.