Gem Air Pistol
Gem Air Pistol ('Made in Belgium', attrib Jean Marck)
With thanks to Steve for the heads up.
John Griffiths, author of The Encyclopedia of Spring Air Pistols, says of this pistol:
It is... a superb example of the very rare Jean Marck Gem, and is probably one of the last ones they made (ca. 1930), which would account for its immaculate condition. This model was apparently made over about 40 years, starting from about 1891, the early ones being stamped with this patent date, and it underwent virtually no changes in its lifetime. The only major differences I can see is that the early guns were not marked “Made in Belgium” and carried only the 1891 patent mark; they had no serial number, and the cylinder end plug had a square hole to facilitate removal.