This is a Westinghouse socket, designed specifically for Westinghouse-based light bulbs, and it is made of thick, heavy brass. It holds bulbs by pushing the bulb base into the socket, rather than screwing into it, where it is held by several flexible clips. It can also accept Edison-based bulbs through the use of an adapter that was made available around the year 1900. Apparently, Westinghouse sockets are much harder to find than Thomson-Houston sockets.
This here is another variant of the Westinghouse socket, made of porcelain. It's even harder to find, and usually sells for more than the solid brass type. It was manufactured by Bryant.