This is the brass junction block for brake lines and the brake warning light switch typically found in the engine bay of classic British cars built from about 1968 onwards. It houses the pressure differential switch, which detects if one brake circuit has failed, and causes a warning light on the dash to light up.
This one is NOS, never been fitted to a car but the box is pretty grubby, as the pictures show. It is a genuine British Leyland part, quite rare and not available as a reproduction to our knowledge. It comes complete with original new switch, which has a chip in the plastic. We do not think this will affect its functionality, and it won't show when the connector from the vehicle's wiring harness is plugged in.