How does this work?

I bought this a few years back and believe it is a connection to fill my model's gas tank.

But I am not sure how one uses it to get the gas into the tank and am puzzled by the centre bit which unscrews and seems to have the pin spring loaded. But not when it is fully screwed in place.

I have a loco and filler extension on order from Roundhouse to suit EN417 gas tanks, and maybe all will be explained in their operating manual, but this is for my Mamod conversion which I showed you earlier. My tank will fit in a housing resembling a coal bunker on the rear of the Mamod.

There is a hole in the side which I guess is redundant because my tank has an outlet with tap going to the burner?