The photo of the Lapp helical design is U-819. Note that the helical turns opposite of that shown in Jack Tod's drawing of U-819. This is an original Lapp photo from circa 1935. Jack made the drawing of U-819 from a Lapp scale drawing for style #6974. The drawing was dated 3-5-29, revised 5-4-31. The drawing was also marked "Pat. No. 1,742,628", which is the Svend Barfoed patent for a post insulator and suspension insulator with side petticoats. The patent doesn't mention a spiral or helical groove. Insulator No. 6974 had a "double spiral" and No. 6973 had a "quadruple spiral". The spiral or helical feature was in the 1932 patent, 1,869,397 by Charles L. Stroup.