Devised in 1866.   36 threads per  inch of the 55° Whitworth form.
The male thread has an outer diameter of  0.7965 inch (between 0.7952 inch and 0.7982 inch).
The female thread, in the  nosepiece, has a top-of-thread diameter between 0.8030 inch and 0.8000 inch.
Thread length 1/8 inch.
(Thanks to Tim Rickard.)