Old disassembly photos show the rubber "gaskets" between the bumper and the fender to be attached to the lower bumper bracket at the bottom and perhaps the grills at the tops?
Sorry, the photos were cropped off at the top of the bumper.
They definitely look like they need to be installed to the bumper subassembly before that is mounted to the car.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI