Cast aluminum fill tube with a screw on billet aluminum cap. This screw on tube is made to avoid the issue of having the oil cap fall off due to vibration or blowby; and spilling oil all over your engine.
The short version of the oil fill tube is for 3406 that have the power steering reservoir on top of the oil fill tube. Here is an example:
Available for:1994 - 2010 CAT 3406E, C15, C15 Acert, and C16 Engines