Huge fog Bowl Insulator U number? other info, almost 15 inches in Dia.

Huge fog Bowl Insulator U number? Any pictures in service, values are about $125+ or - from what I've seen recently 2022? These were used in Dixon Illinois area cement plant along Route 2 as it came into the city.

I thought the insulator used "U" number and it already had the style number on it, A "M" number, M-3900. Thanks to all who wrote.

in service photo.. [id=668082088]