Hi Glenn
Takes a bit of following this one - first look at this link
http://www.ukserials.com/results.php?serial=XS
Scroll down the second column until you get to XS109 - read across and it says that this aircraft became N8626M and then N61457 which is NOT current as a Gnat
However inputting XS109 as a serial into the FAA Database brings up a Gnat
N109XS registered to the Denver Aerospace Museum
Then I found this
XS109 (N61457, soon to be N109XS) David Dulabon, Astre Air International (restoration to fly), Denver, Colorado, USA
Astre Air have a website with a 'contact us' feature (if you want to follow up)
http://www.astreair.com/index.php
Hope the logic of this makes sense !
Terry Fletcher
UK