This is a brass 1/2 inch BSPT Male to 1/4 BSPT male adapter. Ideal for use with the Tesla valve. When used with our M6 to 1/4 BSPT female adapter you convert to 1/2 BSPT.
Please note the UK and many other countries use BSPT threads. The USA uses NPT threads.
It so happens that 1/2inch BSPT and 1/2inch NPT are interchangeable. Although slightly large this makes this adapter ideal for the Tesla valve if you live in the USA.
You should also note that the M6 thread for airlines is also becoming more common in industry worldwide.
More info on threads here http://pipeandhose.com/?q=node%2F2