Hi Martin,
I can but provide you with a detail from Jacob de Gheyn's exercise manual
Wapenhandelinghe van Roers, Mvsqvetten ende Spiessen, 1608, showing a caliverman loading his matchlock caliver (German:
Schützenrohr) from the flask.
First he had to push the lateral cutoff lever against the pressure of a spring, then he would turn the flask uspide down allowing powder to fill the nozzle. Next he would release the cutoff lever, thus saving the correct amount of powder in the nozzle, and hold the nozzle over the muzzle of his caliver. By pressing the long top lever he allowed the measured amount of powder to run down the barrel.
This last step is depicted in the attached engraving, with one difference: as the type of flask shown does not seem to have a long spring-loaded top lever he had to use his thumb to close the nozzle and measure the right dose of powder.
This procedure was basically the same with both caliverman's and (trapezoid) musketeer's flasks.
I also attached an image of the detached top mount of a trapezoid flask and view of the internal mechanism of such a flask, illustrating the thin oval cutoff plate moving on the same rivet, and parallel to the cutoff lever, and closing or opening the nozzle entrance.
The fact that you see two nozzles on the same top mount is due to the special construction of that unique two-way flask:
http://www.vikingsword.com/vb/showth...teer%27s+flask
Best,
Michael