Westinghouse
The Pittsburg High Voltage Insulator Co. becomes wholly owned by Westinghouse circa 1921/22, production of pintype insulators ceased in 1953.
U 280
U 281's
U 298's
U 283's
U 288's
U# not determined yet, possibly a U 300
Examples of Westinghouse markings
Characteristics:

Color(s)
--- dark chocolate brown, mottled dark grey, white, dark reddish brown, dark brown, cobalt, dark teal green, light blue, blue, grey

Pinhole(s)
--- glazed

Pinhole Top(s)
--- slight dome

Fire Rested
--- petticoat

Wet/Dry Processed
--- wet

Marking(s)
---see pics above