Be aware that there are plenty of forged overprints on genuine stamps from Far Eastern Republic and Siberia on
ebay and other online auctions.
Most of them appear to be modern inkjet creations. Here is an example from the Far eastern Republic:


Notice the pattern of dots in the overprint - inkjet characteristics.
Here is an example of a genuine overprint (on a different stamp) Notice the solid typeset printing.


I had previously posted what appears to be an inkjet overprint on a Nikolaevsk on Amur issue here:
https://goscf.com/t/27493#27493