Just tried doing one of these last night
(Kiturin Maru, sunk at 07:30, Dec. 11th, 1942, Bismark Sea)
Used The Gimp, [
www.gimp.org] with the "double HSV film grain" script, then blured it a bit. First time I've used that script so I still have to do a lot of playing around with it before I can get it to look like grainy/contrasty film.
Most Periscope shots I've seen from that era were pretty poor looking. I read somewhere that the head served as the darkroom. Probably hotter than hades in there so I assume the films would tend to be over-developed. ISO 200 was considered super fast film at that time.