Recently, I've come across some 1971 National Geographics that have different back cover ads than what were normally on those particular issues. These issues were probably overseas issues. The overseas January February April and June issues had back ads for Bank of America. The March and May issues had back ads for Minolta. The issues distributed in the USA had Toyota on the back of the January 1971 issue. Mazola for the February issue. Northwest Orient for the April issue. Kellogg's cereals for the May and June issues. I wonder how many other ad discrepancies are there?