It's a nut with a nyloc insert in silver. 5.5mm outside dims.
Very expensive from Thomann for some reason - very much cheaper from Screwfix in the UK, however they don't do the corresponding M3 countersunk machine screws.
Ideal for mounting XLRs in a rack panel. It's tricky to get the nut spinner on when doing this, but that's the fault of the panel and the connector rather than this nut.
Can't really fault these other than they are not black like the machine screws or the panel, but this is not an issue if they are to the back of the assembly.