The felt on my two from Bostick & Sullivan frames is 4mm thick. B&S sells these, but I understand they are built by a neighboring business. Bill Schwab's 7x17 felt is 3mm. The two from B&S have been used very little. The 7x17 has probably been used 600-700 times. This may have flattened the felt a bit. Both builders used black felt.

Your machined knobs are beautiful. Can you explain the frame design you have in mind? On my three the ends of the spring go into cuts or grooves on the interior sides of the frame. To do that with yours, the knob edges would prevent the ends of the spring going into the cuts. How does your design function?