My guess is that the 'slim bumper' is the GT350 version through 2020 which uses the 2015 - 2017 style bumper without the larger plate recess and without the center bump-down, both added for 2018+ on all other S550 versions.
What does "slim cover" mean?? On the 2018+ S550 Mustang the rear bumper covers are listed as either "with slim cover" or "without slim cover". What is that actually talking about?
The Ford HVBOM and parts books for a 2021 Mach 1 call for JR3Z-17K835-APTM "All w/o sensors w/o slim cover...