I just recently purchased 76 volumes of NG with embossed covers of faux leather. The style of cover is reminiscent of a cover that might have been done in the late twenties, given that the plane on the front could be a stand-in for Lindbergh's. I have copies that stretch from 1912 thru 1972 with minor gaps in between. My question concerns the process of the method of reproduction. Were these put together all at once or done after the passing of each year? Given that the earlier years could not have had such a modern plane represented on the cover, at what point did they stop using this style of cover?