From Sunland Produce, a Middle-Eastern/Armenian market in Sun Valley. In Russia, they tap the trunks of birch trees in the spring - sort of like what Canadians and New Englanders do with maple trees up there. The sap is watered down - with genuine Russian water I guess, and exported to the U.S. where it ends up in both quart bottles and weird-looking gallon jugs (think: moonshine jugs). It has a subtle "woodsy" taste, not a bit like the Royal Castle carbonated Birch Beer of years past.

An internet listing for the stuff: https://russiapedia.rt.com/of-russian-origin/beryozovy-sok-birch-juice/index.html

Even shows a picture of a jug! It most likely kept Rasputin Tootin'. Try a Russian market around Las Vegas? Jones Market Eastern European?