I guess I have to throw my hands up in the air. I can't tell from the photo which type of gun it is.
There are two possible orientations for the trap. It might have markings on it indicating which side should be facing forward, or it might not. If it doesn't you might have to try both orientations.
Here is one datasheet for that CRT.
http://tubedata.milbert.com/sheets/093/2/21XP4A.pdf
The fact that the centering is easier is encouraging. Maybe someone else is more familiar with 21XP4A guns.
John