I received a pair of these early third rail insulators in a trade at the YPCIC show this past Sunday. Dan May says they were used on an early system in Boston, Mass. I suspect they were used on the elevated system or the early subway there. I was also told that the top portion that actually holds the rail looks small, so the line was likely a narrow gauge line.

The insulator is comrised of iron and wood and there are a few clues as to who may have produced them. The number "958" is stamped on both the top and bottom pieces of iron. One of the bolt heads show the familiar "Diamond P" marking we see on those elusive CD 134 signals. Both being from the Boston area makes me wonder if there is a connection...

Any guesses?