Another oddball insulator from the lineman's collection. Can't ID the U-number and neither could a porcelain signal specialist. Can you?

It is 3.5 inches tall, 3 inches wide at the base. Has no markings. Skirt straight up and down, double petticoat, recessed inner skirt, the outer skirt is the firing rest, unglazed pinhole with just a faint circular impression at the top of the pinhole, wire groove seems wide for a baby signal and almost square in profile. The lower portion of the wire groove is like a flat shelf. Not MLOD.