Genuine overprints on these stamps are quite rare. Only very small quantities of those were present in the stocks of the postal agencies – if at all. Genuine overprints exist, most of them are probably counter productions or made by Serebrakian.
Here a “nice” lot from eBay featuring exclusively faked overprints.
I agree with Trevor’s comments. For the most part these faked overprints are applied to postally used stamps but I do have a couple with fake Ekaterinodar cancellations.