If you need to fix a lampholder onto wood try this Brass 10mm Wood Nipple.
The idea is that the Brass Wood Nipple is screwed into the wood with just the 10mm fine thread sticking out.
The lampholder is then screwed on and it will then sit flat on the wooden surface.
10mm fine thread to 3/8" (9.5mm) course thread.
Total Length: 15mm
Wood Thread Length: 7mm
Fine Thread Length: 8mm
Wood Thread External Diameter: 3/8" 9.5mm
Fine Thread External Diameter: 10mm
Internal Diameter: 6.4mm