Please refer to the following youtube video (pause it as soon as it starts)...
"USS William D Porter, the Unluckiest Ship in the Navy" by The History Guy https://youtu.be/f9Gb4PakFTU?t=162
There is a running clock super-imposed over the film which gives a HH:MM:SS:FF (where FF is the number of frames elapsed this second, 0-29).
How did 1940's technology achieve this?
I suppose I must concede that this counter may have been added years or even decades later, but the clocks on display here (there are several clips within this video that feature different styles of clocks) seem to "blend in" with the old footage which leads me to believe that they were added (perhaps in post-processing) during the war period.
Given the astonishing ingenuity of wartime engineers I think this would be a relatively trivial task for them, but I would like to know how it was done.