Hi Martin,
As an optimum fit with a ca. 1525 snap-tinderlock arquebus like yours, I would recommend either a powder horn terminating in a mechanism at its broad end, as shown on a series of tapestries of the Battle at Pavia which are now preserved in the Capodimonte museum Naples, a small round flask with top mechanism, as depicted in a drawing of ca. 1520-30, or the earliest type of a trapezoid flask combined with a leather pouch (most certainly not for balls but for cleaning tools that could be screwed to the threaded iron finial (German: eiserner Setzerkopf mit Innengewinde) of the ramrod - the ramrod on your arquebus is not fitted with such a finial, I realize, as it was missing from the original gun that Armin copied):
http://www.vikingsword.com/vb/showth...rapezoid+flask
Tomorrow, I will take good images of small and round ca. 1560 Nuremberg flask in my collection, the obverse also with a nailed-on leather pouch much too delicate to hold balls.
Generally spoken, the mechanism on such early flasks was nearly identical to that on later 16th c. samples.
Best,
Michael