"Taking a pocket flash light from my valise, I placed it in my trousers, so that I could read my watch if I woke up later in the dark." - The Shadow Over Innsmouth, 1936
That line leapt out at me as I was re-reading Lovecraft's tale of a horribly corrupted seaside town over the weekend. My mental picture of his stories is usually illuminated by flickering candles or lanterns, but battery powered flashlights were available at the turn of the century and in widespread use by the end of WW I. By the time of the "classic" Mythos era of the 1920's and 30's there were a plethora of models available, as demonstrated by the incredible archive at The Flashlight Museum .
Inside you'll find hundreds of battery powered lights of every possible shape and function. If you're looking for an absolutely authentic prop for a live action game it's an invaluable resource for identifying appropriate candidates found on Ebay, like this vest pocket light manufactured in 1920 by British Ever Ready:
Or this surprisingly modern looking Eveready model from 1923:
A 1929 model from Bond made from rubberized fibreboard and steel:
And a Utica brand penlight from 1930:
What strikes me the most from browsing their collection is that almost any flashlight without obvious plastic bits seems to qualify as a "good enough" prop for game use. In a perfect world everyone would carry around an $85 antique from 1925, but there's something to be said for using an almost identical $4 Eveready from the 1960's.