David in New Zealand has reproduced exhaust nuts for 1926 to 1933 Norton OHV and OHC engines. The parts books refer to these as "Exhaust Pipe Flange Locking Nuts". There are two sizes, both 14 threads per inch:
Exhaust Pipe 1 7/8"
Nominal Thread 2 5/32"
Exhaust Pipe 1 3/4"
Nominal Thread 2 1/16"
The exhaust nuts are fabricated to the original specification and are finished with black etch primer. The cost per nut is NZ $125, plus postage. Contact David Warren email@example.com