I would guess that most of them were war-time losses, given the Russians were losing about 200k rifles per month due to both destruction and capture. My next guess would be random still-hidden caches in Spain, as the Soviets donated a bunch of arms to help the Communist revolutionaries in Spain, mostly non-standard rifles that didn't fit into the system any longer.
Again, this isn't something in the irrational pile. There's some really interesting history with a lot of the weapons being named in this thread, all with legitimate collectible reasons to own. The Winchester M1914 was a really interesting part in the Russian WWI debacle. When a country is starting the war almost 1 million rifles short of their enterprise requirements that day, not to mention 200k/month combat losses on top of that, it's bound to create an interesting situation where they take in literally anything they can get their hands on, in any caliber provided ammunition came with the gun. They even bought hundreds of thousands of blackpowder Italian Vetterli rifles, not to mention pulling out their own blackpowder Berdans, and taking in French Remington Rolling Blocks in 8mm Lebel, Japanese Arisakas, ect.
The early breechloading-to-WWI arms history is really fascinating to me. Anyone with that itch should scratch it with the youtube channel C&Rsenal.