The item I think you are refering to is not necessarily a non-return-valve but rather an air bleed orifice, it comes from a "y" piece and runs a single line to the header/expansion tank, one upstream leg from the "y" comes from the thermostat housing the other from the top tank of the radiator, simply to pass air from these two high points in the system into the tank and prevent accumulated air locks. I have heard of people using a simple "t" piece in absence of the original "y".
Sorry I cannot supply a photo.