Today's envelope was extra heavy, though. The editors packed in a little surprise: Pocketbook of the Gauge Marks, a guide to the gauge marks found on slide rules by Panagiotis Venetsianos.
Any special mark a manufacturer added to a slide rule for added convenience of operation (e.g. almost all slide rules have a gauge mark for pi) is listed in a compact format. The booklet is 70 pages of 7x8.5", spiral bound. According to the author, this is a condensation of a 600-page French-language book he wrote several years ago.
Example: someone made a rule (don't know who, don't know what) that marked the spot at 1409 with the mark "Pr". According to the present manual, 140.9 is the atomic mass of praseodymium.