With what I think of as pictures on the one wall my conclusion he is inside with hidden sliding walls for the rest of the building. Also I can't help but see it looks like inside because of the mat by the front door. Otherwise when I see him as outside I agree it is a floating house.

How big a sample are you testing - get a 4 x8 cheap plywood (out of the Home Depot reject pile) or several. You are on the right track with the grayed greens I think. Has your house ever been painted with modern paints - even if it has, because its vintage and probably using vintage materials you might want to consult with limeworks.us or ,either and consider using a mineral potassium silicate paint which forms a chemical bond with the cement vs a plastic film which sits on the surface. It is much more durable, breathable - rain is kept out,, but water vapor can pass in and out which you need so moisture doesnt get trapped behind the paint film and cause damage. Because these products use real mineral pigments (vs. chemical colorants) there is a richness and depth to them that cant be matched with plastic paints. No I dont work for them! I just had a good experience with them re: what to do with my own vintage 100 yr old masonry. Beautiful house btw!

I had to leave them and go do something else, come back much later to make them upside down again. Once they were all up, they were up. I tried squinting, looking away, refreshing the page and couldn't unsee the upright-as-would-be-used position until just now.

Need to get a sample and test it for yourself. It is a naturally occurring product so not all stone in the same family will react the same.