This is a very nice clean Screw Thread calculator copyright in 1935 and marked "THE LUFKIN RULE CO. SAGINAW MICH. U.S.A. COPYRIGHT 1935". Also included is a decimal equivalents chart, that also includes decimal sizes for number and letter drills, and fractions. The calculator is for finding sizes of tap drills for NC and NF, also NPT on the back side, & deciamal equivelents for fractions. They both are printed on a cardboard, the calculator is two piece with a brass center pivot in good condition. Although these were printed for a number of years, this type has a clear coating on over the printing which is fully intact. These two Lufkin advertising printings have been helpful machinist information charts for many years. These two are in very good condition, some minor yellowing, and the rectangular chart has very little minor edge wear also. These are hard to find in this condition!