@Rob13
This might ease a bit of your anxiety if not cured it. I believe Russian stamps do have some tricky answer with regards to the inverted groundwork watermark but Cherry Auction house seems to be answering the $$$(values) of each inverted groundwork watermark ( it's not only 4K & 10K).To my surprise the answer might be"as long as it's meet the requirements" as our friend @vasla from quoted.
Here is the link that might help you
https://www.cherrystoneauctions.com...untry=RUSSIA