What is extra part in Fuel Pres Reg. kit?

I have a 2000 silverado with the 5.3. It has trouble starting when warm, and gets gas in its vacuum line, so I got the fuel pressure regulator kit from the dealer. The parts guys told me that the part # had recently changed, and that the price had dropped from $90 to $50! In addition to the regulator body, gaskets, screen, and spring clip, there is an extra part that wasn't on my truck originally! It is a thin brass-colored, horseshoe-shaped metal gasket that looks like it is supposed to go between the FPR body and the fuel line. I tried, but could not get the new FPR on with this extra part. The parts dept. guys told me that if it wasn't on the original, just throw it away. All of the other parts in the kit are identical to the original FPR. What is this thing?

most of the times those kits are for many different trucks so there are going to be extra parts some times