This is a Westinghouse socket, designed specifically for Westinghouse-based lightbulbs, 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. I was able to get this socket for a very low price ($20), but these usually sell for more. Apparently, Westinghouse sockets are much harder to find than Thomson-Houston sockets.
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.