Hi Yancey,
And welcome here!
All iron mounts (not just the top mounts) on trapezoidal musketeer's powder flasks were either tinned or blued/blackened. Anyway, they were never left unfnished in a polished-bright iron surface originally - but unfortunately most museum's and private 'restorers' have tended to scrub off every tiny bit of original surface from old iron surfaces, just to present them in an unnaturally 'shiny white' ...
You will also be interested to learn that they were not only confined in time to the late 16th c. The earliest trapezoidal flasks are recorded from paintings and in collections (including mine) since at least the early 1520's (!) but seem to have come out of use by the beginning of the 17th c.
The earliest flasks had a height of ca. 16 to 18 cm, their latest forms of ca. 1600 were ca. 20-24 cm high.
Priming flasks of course were smaller, about ca. 11-13 cm.
Anyway, my following threads and posts should be able to answer your questions satisfyingly:
http://www.vikingsword.com/vb/showth...sks+bandoliers
http://www.vikingsword.com/vb/showth...d+flask+munich
http://www.vikingsword.com/vb/showth...s+powder+flask
http://www.vikingsword.com/vb/showth...s+powder+flask
http://www.vikingsword.com/vb/showth...s+powder+flask
And please DO use the SEARCH function on the forum: enter tags like 'trapezoid(al) flasks', 'musketeer's powder flasks' or simply 'powder flasks', 'bandoliers', or 'matchlock/wheellock musket', 'landsknecht (h)arquebus' to learn about the guns used together with these flasks - and you will find a lot of stuff for your studies!
Best,
Michael