This is a very nice clean Screw Thread calculator copyright in 1935 by "THE LUFKIN RULE CO. SAGINAW MICH. U.S.A. COPYRIGHT 1935 L.R.CO." The calculator is two piece, the top round section spins, for finding sizes of tap drills for NC and NF, also NPT on the back side, & decimal equivalents for fractions. They both are printed on a cardboard, or heavy paper in good condition with some slight age toning or darkening. The calculator has a brass center pivot in good condition. Although these were printed until the 1950's era, we believe this one to be from the 1940's. It measures about 3.3" x 4.75" with a 2.75" diameter round section. It's in very good shape! Also included is a bonus decimal equivalents chard with a letter size drill chart on it also. Also in good shape for about the same era.