funky nuts

i replaced the muffler on my 71 911s with a dansk stainless steel sport (2 inlets one outlet) and since the nuts & bolts that attach it to the heat exchangers were very rusty i ordered new ones from a dealer. the bolts seem normal enough but ive never seen any nuts like these (#999.084.002.02). they appear to be copper and have a sleeve of sorts that extends from one side of them by about 3mm. there is a slit cut into the sleeve and then that gap is pinched closed. althought this sleeve is threaded along with the bolt its is not easy to thread the bolt through. i am surmising that this is some sort of funky locking nut with the sleeve side of the nut going on the end of the bolt.

does this seem right?

Although I have not seen these particular nuts, these sound like mechanical lock nuts. The "sleeve" end you describe is the locking portion of the nut. It faces away from the clamped assembly.

Sherwood Lee