As I understand it, during that period the Leica M mount patent was still enforced. There were no third party M mount lenses then. I doubt Nikon would have violated the patent.
There were Leica M Visoflex adapters for the Kilfitt KI/AN/WE, Novoflex Follow Focus / Nesting lens, and Komura Uni lens systems and also for simple LTM to LBM adapters made by the same manufactures. I assume these were approved by Leica.
What is really puzzling is the unique serial number sequence and the span of serial numbers. If that many lenses were made, they could be frequently be seen.