Mystery valve help needed

My son's recently acquired '90 YJ (4 cylinder, 5-speed) has been
smelling of gasoline, and a little checking revealed a broken valve
tucked up inside the body behind the right rear wheel well. I have a
factory service manual and have studied it carefully but can't figure
out the purpose of this valve. It looks to be a check valve, on one
side there's a line going to the fuel tank and on the other side
there's a vacuum hose that runs up to the engine compartment. I
thought mabe it was a rollover valve but according to the manual those
are supposed to be attached the fuel tank.

What do I ask for when I go to the dealer to get a replacement?