A system using a 3/8" thick gypsum panel (16"x48") was in use at least by 1930. It was used in place of wooden lath, and received a three-coat finishing system, ending up with a 3/4" thickness. Drywall finished only with taped seams is a bit later, like post war, The adoption was a slow process, and rocklath was only completely phased out within the past 5 years. Veneer plaster systems("blueboard") were introduced in the 50's, and finally surpassed and replaced rocklath as the premium wall finish system. Reducing the time factor is key in every newer system; rocklath was faster to hang that wood lath; Veneer plaster is even faster, as there is no delay needed between the brown coat and the white coat, and of course drywall requires the least time and can probably be accomplished by trained monkeys, from what I see on HGTV ;-)
Casey

Casey, thanks for the info! The transition started a lot earlier than I had thought, although I think it may have been a little later here in rural Minnesota. We've also come across what looks like two different types or grades of plaster while removing the many layers of wall paper. Some is somewhat dry and crumbly, as I had expected from old plaster, and the other is hard as concrete.
We are replacing a wall that some PO in the past had removed and patched the plaster where the wall had butted into the remaining wall. We want to remove the patch so as to tie into the studs properly but we're having trouble chiseling out the patch. I'm not so sure they didn't use QuickCrete or SackCrete!
My wife and I got a good chuckle out of your comment about drywall, trained monkeys and HGTV! Nice one.