Of course, Markov is a worthwhile resource, having auctioned a Constantine Ruble in The New York Sale several years ago.
Jim Elman, World Wide Coins of California, is another long time Numismatist and dealer, and has been very active in the Russian Numismatic Society.
He has sold many famous collections such as those of Brekke, RW Julian, and many others. He has seen it all. Perhaps more than Markov.
He is in Santa Rosa California. I can pm to you his tel. and email.