Carsflash has been working on his techniques for quite some time - he is getting closer all the time.
He is taking later (1980's) 309 castings which don't have a thumb pad..., machining out a pad, and stamping the US.
Then he drills them for bleeder valves, and installs a deep cylinder. - Pretty close - but not exact.
The size and shape of the pad is different, as well as the stramping.
He does have access to real ones too - just ask him, he has been pretty straightforward with me.