British Standard Brass is used for brass tubing, gas fittings and general brass work (quoting from machinery's handbook here).
It will be 26TPI, Whitworth form. Easy enough to make on the lathe if you've got the wherewithal. Whether you can find one off the shelf that easily is another matter.
If you just need something with a 7/16" x 26 tpi male thread on it to plug a hole, seek out a suitable BSCY (or CEI) fastener. PTFE tape is your friend and the 5 degree thread angle difference helps it to seal better.
Bill, if it's any use to you, I found this hiding in a piece of brass offcut:
The threads are about 1/2" long, 0.437" OD 26TPI, the flats fit the spanner it's pictured with. If you want it, PM me with an address and I'll drop it in the post.