Dieseler with a spark plug question

[CzarFC]

What are these little round barrels with threads used for? On the spark plugs I pulled out of my 560SEL, these were not attached nor are they in the spark plug cables. I have a sneaking suspision that they are threaded to the top of the spark plug and are what the cable snaps onto, but these are the first spark plugs I've ever dealt with ever, and I'm 26 years old. Was the prior wrench turner on my car further showing their incompetance by not having any of these on my old spark plugs???

Thanks for the info!


Scott