Actually, it is a completely different base form. Though similar to the Medkit, it's not a cut-down version but rather an entirely different vacu-formed shell (HMS Mike posted a pic over at TPZ). It's noticeably shorter than the Medkit, but wider.
One thing to note regarding the dimensions written in that auction description. They're off by about 1/2" inch all around (height and width), based on the measurements Mike provided in that pic of the raw vacu-formed pull Nick posted above.
I drew up a revised set of templates based on those dimensions from Mike some time ago. I'll see about posting a pic of them. Planned on building one of these up after I did a new and improved build of the TNG Medkit I have in my collection