Adapters range from A1 - A7, sizes are given in BSP (British Standard Pipe) - Example 1/8" BSP is not the diameter of the adapter.
1/8" BSP (British Standard Pipe) measures approx 9mm diameter.
1/4" BSP (British Standard Pipe) measures approx 12mm diameter
1/8" or 1/4" BSP MALE = External threads
1/8" or 1/4" BSP FEMALE = Internal threads