It appears that the viewfinder worked by viewing the image on the iconoscope mosaic plate. The operator apparently needed to keep his head in place to block stray light.
1936-1937 441 line camera003 by
old_tv_nut, on Flickr
With the cover closed, and the notations on the back of the photos:
http://www.flickr.com/photos/4200284...7640097777643/