Russ Frank recently reported a porcelain Surge electric fence insulator with drip points [id=170540955]. This evening I found the patent to that insulator, which was assigned to Babson Bros. Co. Patent 2,350,420 was assigned on June 6, 1944 to L. J. Schilling and states the purpose of the drip points: "It should be particularly noted, however, that the bottom of the insulator is provided with a quite deep depression or upwardly extending pocket 24, to lengthen the leakage path from the bottom of the skirt of the insulator to the mounting bracket on the dry underside; and that the bottom of the skirt has been provided with a number of projections 25 which provide drip points and which enable the entire underside of the insulator to be properly glazed." See this patent at

http://reference.insulators.info/patents/detail/?patent=2350420&type=U

You can also read more in Schilling's earlier patent

http://reference.insulators.info/patents/detail/?patent=2311779&type=U

and his design patent

http://reference.insulators.info/patents/detail/?patent=126722&type=D