Robert - The stock brake light switch should be 1/8" NPT which is available at local auto parts stores. I got tired of replacing the cheap brake light switches on my '64 MGB (interchangeable with the Birkin switch) so took the advice of a mechanic friend of mine and replaced it with one from a Harley (I think it was a late 90's Heritage or Road King model). It was pretty expensive at appr $19 back around 2000 but I've never had to replace it since and it takes very light pressure on the brake pedal to actuate. On the Birkin, I replaced the hydraulic switch with a GM adjustable mechanical switch mounted at the pedal since I was going through the whole wiring harness and didn't mind revising the brake switch location.
The H-D switch that I bought looks like this but with a black body: