Regarding the talk about the grooved base on the Yukon and Gisborne beehives. The grooved base is an old feature, dating back to the earliest threadless insulators. Litterature shows that it was used in Preussen (Prussia) in 1853, and on other german iron clad insulators from the same era. The photo here shows an old European threadless insulator with a grooved base, from my collection. The country of origin is not exactly known, but it comes from a technical school in La Louvière, Belgium, where a number of insulators were used for didactic purpose. Sorry, no u-number on this one...I suppose.