This is an 'acorn' or 'dome' nut for a Gibson scratch plate/pick guard bracket.
It is made of Stainless Steel and so will blend OK for Nickle and Chrome hardware. Note this nut is for the newer version. This has a 6-32 Thread.
Tip: As these are frequently lost, it is worth super-gluing it in place and using the threaded rod to adjust to cater for the correct bracket length. Once done, the internal (normal) nut can be 'pinched' up to complete the task.
Please be careful when ordering due to there being differing threads.
NB: Apparently the older ones have a finer thread so this will not work for those guitars. I suggest a UK BA equivalent thread might work. Someone has quoted that "Gibson USA will ALWAYS sport SAE 5-40 threaded parts, so if this size is not specified, steer clear." this is not true as I have sold this unit listed here for the new version of the 335 which originally has more of a flat ended cap.
smaller thread 5 - 40 UNC 0.125 inches 3.175 mm
larger thread 6 - 32 UNC 0.138 inches 3.505 mm