When I googled OTK, I read that not all parts had an inspection mark and not tested, but they all had a number stamped on it. This may be why mine had a mark and Sean's doesn't.
Copy and paste- these departments were inserted in most industrial companies of the Soviet Union to test and check production. There is always a number with the OTK, and that is controller's ID/department number.
Copy and paste- If an item bears the stamps, it has met minimum quality standards. Not every item that leaves the factory will be stamped, and not every factory will have a OTK controller embedded.