This is a U-746 found only in British Columbia. It is marked K1 on the base of the outer skirt. These are rarely marked this way, and sometimes also hand scribed 91075 on one ear. The markings are unattributed. See next photo for the K1 marking. Paul Axman just informed me that this marking and insulator are almost certainly attributable to early Canadian Porcelain production (the company was known as Canadian Insulators at the time). He has a CP insulator marked J1, which seems to support this identification. Thanks Paul!