U-Nut
U-Nuts
U-Nuts for self tapping screws No8 (4.2mm) ... No12 (5.5mm)
I try to translate what I red some times ago in an internet forum which describes the U-Nuts quite simple:
U-Nuts are small metal clamps with a hole where you can fix something. Mostly used for fixing loudspeaker in cars. The U-Nuts are clipsed onto the metal, then the U-Nuts are used like standard nuts.
Many Thanks to the unknown author who gave a quite precise and simple description.

Another theme is rust. The manufacturer claims the following data:
Salt Spray Protection for Phosphate Delta Tone Black: 250 hours minimum
==>This means better then standard passivated material which shows a resistance of 72 hours.

Beside these data I would recommend a standard rustprotection like Waxoil, Dinitrol, or paint.
(These are brand names and are the owned by the relevant companies)
!

U-Nuts No8; for Sheet 0.7...1.6mm;

 : 
BM0816
1Packet=5pcs

U-Nuts, No10; for Sheet 0.9..2.1mm;

 : 
BM1021
1Packet=5pcs

U-Nuts, No12; for Sheet 0.9..2.6mm;

 : 
BM1226
1Packet=5pcs