(set in 1920?, written in 1920, published in 1934)
The picture was very vivid for a while, but gradually gave way to a more horrible conception; that of utter, absolute solitude in infinite, sightless, soundless space. There seemed to be a void, and nothing more, and I felt a childish fear which prompted me to draw from my hip pocket the revolver I always carried after dark since the night I was held up in East Providence.
The unnamed narrator habitually carried a revolver in his hip pocket. This indicates a fairly small and light design, with a short barrel and probably in a small calibre. Given the timeframe, it was likely a double-action revolver. A savvy user would acquire a hammerless design, which is both safer to carry and quicker on the draw from a pocket, but of course we do not know whether the narrator falls into that category.
Although there were also many other options available at the time, two suitable designs spring immediately to mind: The Iver Johnson Safety Automatic (Investigator Weapons 1: The 1920s and 1930s, pp. 47-48) ‒ a revolver despite its name ‒ and the S&W Safety Hammerless (Investigator Weapons 1, p. 57). Both were available in .32 S&W (7.9×15mmR) and .38 S&W (9×20mmR) and have features that allow them to be carried fully loaded in complete safety, in defiance of the cautionary notes about loaded revolvers in most editions of Call of Cthulhu. The Iver Johnson was cheaper ($5 in 1920) and more common (5 million made), while the S&W was the older and more distinguished design ($13), but not quite as widespread (500,000 made).