The mounting thread of this lampholder has an internal locking screw to ensure secure fitting to the mounting thread.  Another benefit of this lampholder from a user perspective is that the porcelain insert has terminals for live, neutral and earth allowing ease of wiring. Anyone who has tried to wire up a lampholder where the earth terminal is in the body of the cap will understand this is quite a relevant point!  Solid machined lampholder just looks the part.  You do need to keep an eye on the locking tabs and adjust (with a small screwdriver) when screwing the lampholder together.  Fitted and assembled correctly this lampholder will outlast almost any other on the market.  Fits a 40mm shade hole

If in doubt consult a qualified electrician