These are the known print references to it. The earliest was taken from a 1911 Heiser catalogue:
This one is 1918:
1929:
Above, that comment 'lay it aside for an hour' is called 'casing': the leather permeates the entire thickness of the leather and begins to dry on its surface, which makes the leather clay-like for moulding. A sought-after condition for leather engraving.
Below: 1956, old Charlie didn't know what he was talking about -- wetted for SIX HOURS?! Not even for six minutes: production holsters do quite nicely with THIRTY SECONDS maximum in hot water. What temp? Not so hot as to scald you will do nicely!
1959:
Notice above what 'boning' really is.
1966:
Notice above how early in the century it was worked out that 20 or seconds' immersion is long enough; but use hot water for (1) quick penetration and (2) cause it to begin to dry straightaway. Then notice the switch from greasing to a plastic bag: such materials were NEW then. Doubt I would use cellophane but likely the author meant polyethylene such as food wrap.
1968:
We've reduced the process to a science since these old articles; since the 1960s a production maker uses a rubber-padded press (except Galco but including vintage Bianchi, Sparks and Safariland leather companies) to squash the holstered mould inside the wetted holster). Sparks:
Above, the plastic bag you see in the press is around the wet holster not the mould that's inside it. It's used to prevent the rubber pads from leaving pore marks in the leather under all that pressure; quite ugly otherwise.
Dried in hot air including at Bianchi, for example (below), but not at Sparks which company never did figure out that hot air is required to soften the leather's collagen to harden into its new shape while heated. I use about 50C for a half hour; if your holster then becomes too tight for the mould you used to shape it, then lower the heat of your hot air dryer until the fit is 'just right'. NEVER dry a wet holster in a closed oven of any sort, you'll get a 'prune'; assuming a box shape such as a fan-force oven, leave the door open for the hot, wet air to escape. Below, vintage Bianchi holsters were dried on racks that passed through an open-ended hot air conveyor arrangement: went in wet, came out dry, in the manner of a pizza oven.
Above, not really ready to pack: they're dry but they haven't been completed by any means. Notice all the empty holes for hardware!
I use a simple box with a fan-force room heater blowing into it through that open 'door' or side. This one is a 1-foot-cube rattan drawer of all things, and the thermometer shows the ACTUAL temperature inside the open-sided box vs. what you're hoping for.
There also is a truly secret method of detail moulding the wet leather holsters, but I didn't create it so I won't say more than that about a process I've seen at three makers so far. Clever though. Doh! But trade secrets stay that way, with me, always have (never did tell anyone how Safariland's Kydex holsters were formed in their factory, in the '90s).
Read more in my book titled "Holstory -- Gunleather of the Twentieth Century -- the Second Edition" that is available at www.holstory.com and printed for you/shipped to you in USA.