"For S&W, mentioning on the barrel that it will also fit the older .38 LC, which at the time (until about 1909) was the standard US military cartridge for the DA Colts, was pure marketing."

I probably wouldn't say it was there purely for marketing purposes, as the .38 LC cartridge was in wide usage at that time in both military and civilian revolvers (mainly Colts). I'd say the S&W dual caliber barrel stamping was more educational than opportunistic, letting potential purchasers know that either cartridge was OK to use in the Model 1902. The .38 LC revolvers hung on for quite a long time in military service even after the .45 ACP was adopted by the US military, and some guns in that caliber saw rear-area service throughout WWI, and for several years after WWI by the various state guard units. I don't know when the .38 LC cartridge was discontinued by the big loaders, but I remember seeing boxes of it on store shelves in the late 1970s, so it retained some market demand until at least then. I believe some of the specialty loaders still offer it on a limited basis for applications like CAS shooters.