If each one has a date stamp indicating its date, and the badge do not look tampered with, then don't you have the dates? The serial numbers are only 7,000 - 10,000 apart. I would not doubt the date stamps for this relatively close grouping of serial numbers. It appears that you are looking at Rob Cook's serial number guide, which is reprinted at the Ludwig site, among others. If you read the guide in the context of Rob Cook's book, you will see that the ranges are just approximations. Even then, he has 1971 serial numbers going up to about 895XXX and 1972 serial numbers starting at about 896XXX. That would make one of the drums from 1971 and two from 1971. I would not recommend that you read that level of precision into the dating guides.Can you provide pictures of the date labels and the exact serial numbers? That would help.