Here's reverse side photo showing the crown straw line created at factory that was glazed over (not a crack). Below is a bb glaze ping and surface glaze crazing that is highlighted by sulfur that was used in pinhole.

Rare insulator to find! Shown in circa 1915-30 British General Electric Co Limited Catalog [id=612324951]. Slashed price of only $325 plus 9.00 shipping. Thanks!