Klaus, I wonder whether you've got information on how Neumann is administrating the serial number check, or do you know someone whom we can ask?
If someone supplies Neumann with a valid serial number upon request, do they simply trust that the person actually owns the microphone, or is some proof of purchase required?
If the former, I'm not sure much would stop a determined DIY enthusiast from simply "borrowing" the serial number of a perfectly-functioning mic (owned by themselves or a colleague... or even an eBay/Reverb photo!). But the requirement probably would prevent someone from ordering several hundred capsules at once for a production run (which may be the only real goal here?)
A proof of purchase requirement could make things tricky with older vintage mics, particularly those that might have had parts swapped out over the years. One of our U77s actually has a body tube from a U67 (it came to us that way) so determining provenance could get messy.