Thank you so much, Alexander, for posting these!
I fully agree with your opinion and the dating which is clearly supported by the style of the Gothic armor (ca. 1470-80).
Also the existing arquebus that you are referring to (in the Vienna armory) is indeed very closely related to the one in the drawing. It is known, though, that the snap tinder lock serpentine on the Vienna gun is a modern replacement.
Well done!
Best,
Michael