The Trumbull Electric Mfg. Co. Plainville, Conn. were known for their safety fuse boxes and knife cutoff switches. Like Square D Co. though they had a porcelain manufactring plant and made their own knife switch bases, light fixtures and insulators. Square D had a small section of their own insulatorsplus made others for companies under contract and so did Trumbull. I had a split nail knob and some knife switches plus light fixtures made by them but no other actual insulators. I recently found a large strain made by them marked with an ink stamp circle T. Also while researching my radio strains I found an unmarked strain that matched an ad I found from them for a radio strain. Here is a picture of the three Trumbull insulators I have. The nail knob is marked with a recessed embossed circle T also.