The socket weld flange (SWF) is similar to a slip-on flange, except they have a bored and counter bore. The counter bore allows the pipe to fit into the socket/counter bore. The bore of the flange is the same diameter as the inside of the pipe. These flanges were first designed for small diameter, high pressure pipe. This flange has a raised face.
This is the size differences between a 150 to 2500, 4″ socket weld flange (SWF).
ASME B16.5 are flanges for sizes 1/2″ to 24″.