Rich, if you are looking to acquire a (black and white, not black and silver) BMC-XX tag I can't help you, but I can confirm that the tags were only on cars delivered to California, at least based on the evidence in the BJ8 registry.
Apparently, it was a California requirement that the manufacturer's name and the year of manufacture be identified somewhere on a car. Since the BMC VIN did not have a code for that information, the BMC-XX tags were applied to the firewall. Many I have seen were attached with one of the VIN plate screws.
I do not know if the tags were attached before the cars left the factory or if they were applied by a distributor or the individual dealers in California. The tags all look alike, so they appear to have come from a central source. Some photos of examples have been sent directly to you.
Another California quirk is that the two-digit year of manufacture (or "model year", which can be different)was prefixed to the factory VIN on registrations and titles, which has caused problems for cars that were later sold out of California when the paperwork VIN did not agree exactly with the number on the VIN tag.