Lister domestic water pump air chamber bleed screw assembly.
A brass alternative to Lister part number 187-00366 available in three sizes.
The 1/8" BSP is the standard factory size, the 1/4" and 3/8" BSP options are larger than standard and are intended to be used to repair an existing air chamber which has damaged threads. The casting can be drilled out and tapped to suit one of the larger sizes.