I've never made that kit but it sounds like that's the nib cover used for shipping protection. It's not part of the kit, just to keep the nib from being damaged.
I know on a couple of kit I have gotten from CSU, it is actually the part that the Nib snaps into within the cap to hold the cap on when its closed. If you have instructions, they should show you if its needed in the assembly.
The El Toro is a screw cap fountain pen. It is not a CSUSA kit although it is very similar to the Little Havana they once sold. The white part is protection for the nib and is exactly like the part used in snap cap pens. It is not used for assembly....if I recall correctly. I make them but use longer tubes and make them regular fountain pen length.
Do a good turn daily!
Don