ACDelco 29535617 OE Allison Spin on Filter Magnet.
For 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019
GM 6.6L Duramax LB7 LLY LBZ LMM LML L5P

Allison installed a disk type magnet that mounts on top of the external spin on filter to catch metallic debris. Its very common for this magnet to get overlooked when servicing as it tends to stick to the top of the filter, only to get thrown away with the old filter.

This is the factory replacement piece.