The closed helmets were slowly abolished in the 2nd half of the 17th century and the Zischägge was much more common then. The caption of the picture - wich you of course can't read - also says it shows a Curassier. With a Dragoon the gun should be longer so it would be more effective in the dismounted fight. And it probably dosen't show a man from Brandenburg or Hessen as the artist was mostly concentrated on southern german military like Bayern and Österreich. Other similar pictures of 17th century horsemen by Anton Hoffmann even say the specific regiment in the caption. Of course the pictures were made over 200 years after the showed time period but the historic artists in 1900 were pretty good with uniforms...
Well yes that makes sense, I was thinking about the first half of the 17th century, but it definitely makes more sense for the second half.
