Stainless steel coupler or "reducing socket". 3/8" BSPP x 1/4" BSPP. Made from stainless steel.

BSP vs. BSPP
BSP (or BSPT) fittings have a tapered thread and require teflon tape to form a seal between the male and female fitting. BSP fittings are generally intended to be used with their counterpart of the same thread type. ONLY 1/2" BSP fittings are compatible with US-standard NPT fittings of the same size. BSPP fittings, on the other hand, have a straight thread, require a gasket seated inside the female fitting to form a seal, and are not compatible with NPT.

KegLand Part Number: KL10597