This is a huge newly reported 15" Thomas M-4452. It was used on an 8 1/2 mile transmission line in Massachusetts and maybe have been the same Thomas style used on a line in Italy. So far this seems to be the only one known. Dimensions are 15 - 10 - 11.5 - 8 x 15.
In 1911 the Amherst Power Company and Easthampton Gas Company built a unique 66,000-volt tower line in the Connecticut Valley. Energy was purchased in Amherst from the Turners Falls Power Company and delivered to Mount Tom, where an auxiliary steam station was built and substation stepped the voltage down to 4600-volts. Steel towers on the line were very unique with the apex of the circuit pointed down and had a large copper rod installed instead of a static line for lightning protection.
I bought this insulator from Greg Garrison [id=495543888] back in March. We both thought it was an Ohio Brass piece until obtaining it at Springfield 2017, where Mike Spadafora confirmed it to be a Thomas. Thanks to Zac Mirecki who transported this piece from CT.