73-87 brake rotor anti vibration gasket?

[Ghost's68]

I'm in the middle of rebuilding my front end and I'm at the point where I'm working on my brakes. I just installed ECE drop spindles. I was reading their instructions and saw they mention to install anti vibration gaskets on the spindles. I have never heard of these. I can't find them when I try to google search. I bought new rotors from Napa and before I get the brakes on I was wondering what the deal is for them. The old spindles did have them installed though. I'm just a bit lost on this. Thanks guys