a metal funnel and cap
There was a similar device on the Shackleton. Since the Shack had an Elsan, it was allegedly for use if a pilot was taken short during Action Stations at low level and could not go aft.
The only time I was one used was during the Majunga detachment. It was mounted on a wooden plinth, stuffed with issue condoms and presented to the member of the Sergeant's Mess, deemed "Animal of the week". One F/E won it 6 weeks in succesion.