Most likely it's a rebuilder tag. Does it have the Ford logo on it anywhere or just a plain brass tag? The only factory tags I've seen rivited to the side of the block were mostly industrial/irrigation applications. They used a Ford tag with engine CID and specs with a serial number. Being they weren't installed in a car or truck they used a ID tag on them in most cases. Fred Jones (?) was one of the larger Ford rebuilders back then. Also Ford had a rebuild plant in Fairlawn NJ I used to order stuff from back in the 70's. Then there are all the other independant rebuilders that have been around too.