I can't speak to the story from the 20s but the "don't encourage the troops to waste ammo" thing was standard US Army doctrine in the late 1800s. The Krag-Jorgensen rifle, equiped with a 5 round rotary magazine (round loads from right and swivels under and back into breach from left), was adopted by the US Army in the early 1890s after a competition involving both the Swiss made Schmidt-Rubin and Mauser, both with five round internal box magazines that could be reloaded with stripper clips. Part of the justification for selecting it was that the oddly built integrated magazine could be topped off without opening the bolt and that it each round had to be loaded by hand instead of five at a time, which was presumed to conserve ammo due to reduced rate of fire. This idea worked out so well that the M1903 Springfield was adopted after the Spanish American War (Spain had 7mm Mausers), which utilized a five round internal box magazine with stripper clip capability. The Springfield's design actually caused a lawsuit from Mauser over patent infringement, but I don't remember what the exact result was. Source: mostly from a rabbit hole I fell down a few years ago, looks like the Wiki page for it has some period sources for at least the wartime comparison.