I contacted Juergen Malrik to ask someone more qualified than myself about Herr Ziilly and this is what he had to say.
Ziilly, pronounced 'Feelly', is a common low Bavarian family name. Low Bavaria being that western portion which is away from the Alps. The Ziilly family included several gunmakers who operated in Munsigen, which is west of Ulm and Augsburg, during the period 1680 - 1730, with 1710 being a reasonable estimate for the gun listed at gunbroker. Hans, Marcus and Johann Ziilly were prolific gun producers of this family.
Beneath the frizzen was a common place for them to leave their initials or mark.