I haven’t seen enough pre-1960s british documentaries to say how widespread this might be, but the use of voiceover in Under Night Streets, a 1958 documentary about the Underground’s 800 night shift workers, to create a narrative over the pictures seems to be straight out of Billy Wilder’s 1950 Sunset Boulevard.
Not only does our gruff-voiced narrator describe what’s on screen to the audience, he also gets to narrate his own on-screen actions while he’s in vision. How Noir is that?
Two other things that interested me: the film seems to have been shot without sound and dubbed in its entirety after the fact (not very documentary). Secondly, the group of women who ensured the tunnels were free of paper, dust and other flammable detritus were called fluffers. Not like your modern day fluffers then.