Without a word of a lie, I've had that exact conversation, pretty much word for word, with a stores clerk before over a gas mask once.
I'm talking about the sheet metal tail piece you attach to the actual grenade that was never seen. Remember the old HCL smoke grenades with the cylindrical sides and the rounded bottom. They were designed to fit into the grenade launching fin adapter that then slid over the adapter/front sight. Here's a photo from someone else's Photobucket gallery:
I remember reading the old C1 manual where it talked about using the grenade launcher. Turn the gas plug around, load one round of the launching ammo (think really really really big blank round), put the butt of the rifle on the ground, pull the pin on the grenade, sit the rifle at an angle and pull the trigger. I can always remember the description that if the adapter fails and the grenade safety leaver flies off, the drill was to throw the entire rifle as far as you could and hit the ground. Man would I love to see the look on the QM's face when trying to return that rifle to stores. Bet they'd still make you clean the damn thing first.
And you're quite right, the poster is awesome.