I still believe all bikes that left the factory were matching numbers. There were exceptions of course. I know works riders would have had access to whatever they needed to keep them going. The bikes in the uk came in through comerfords. In north america they came through the bultaco importer in new york. From there I think they were shipped throughout the states. There was another warehouse in california that distributed bikes also. If you have a bike with non matching numbers I'd bet it was changed at the importer or dealer level for some unknown reason. Remember Franco era spain was a tough economy to get around in. Bultaco sublet out alot of their parts manufacture etc etc to get around tax laws and duties and such. The government of the time kept strict control of the economy and they knew what was coming in and going out of the country. Bultaco would have been under the microscope when it came to having matching numbers.
Steve