Sounds like it's a directional sign meaning "The Base (Strong point, maybe Command Post) is (in) the Tunnel". I suppose that it could mean a dugout, bunker -- some sort of underground place.

I appreciate the help! This is going to be part of a display for Veterans' (Armistice) Day. A sign with it will read "At the 11th hour, Of the 11th day, Of the 11th month, The guns fell silent. [spaces] The sun's shining down on the green fields of France;/The warm wind blows gently, and the red poppies dance./The trenches have vanished long under the plow;/No gas and no barbed wire, no guns firing now. -- Eric Bogle"

There will be an (American) Enfield P-17 rifle and a German officers saber resting on an Army blanket of the period, some fired 30-06 cartridge cases, the sign I've asked about, and some rusty barbed wire. These in a horizontal display case.

Other items -- and American "flat hat" helmet and a gas mask, a German helmet and a gas mask, a cartoon -- will also be displayed in a vertical case.