As stated in the last several posts, it is a filler tube checkball and when it's in place you will find it impossible to use a siphon hose below 1/2 full!
That ball normally moves horizontally in it's tube, left to right, so it's really a filler-neck anti-surge device
In order to fully drain the tank before dropping it, you must pull the fill and overflow hoses off the tank fittings. Next, if you look closely, you will notice the end of a slightly smaller plastic tube sticking slightly out of the tank filler neck. You may need to grab it with pliers to pull it free (nothing will break) If the section where the ball nornally goes is broken to the point of not being able to contain the ball, then the only purpose it's gonna serve is to still somewhat minimize left surge to the filler neck on right turns.... just not as much as with the ball.
Once that ball tube is pulled from the tank, you can drop in a fuel hose hooked to a pump and suck the remaining fuel out (If you pulled the hoses with more than 1/2 tank of fuel, the top half ran out on the ground!)