Is it possible that you have acquired a fork that should have been tapped with threads and wasn't, and that is the reason it doesn't take a threaded clevis pin? If that were the case it seems like one could try tapping the threads into the large hole and use the threaded clevis pin.
The TR4 clevis pins on my cars all have just a single diameter fork and clevis pin with a cotter key at one end. The TR3 had the large threaded end on the clevis pin.
One last thought is that the hole diameter in the pedal extension is likely the same size as the small hole in the fork ( unless it is excessively worn). So the large hole is useless unless it were for the threaded pins.
What about using your old fork on the new master cylinder? When I am confronted with nonconforming new parts I often will reuse some old parts. In this case the old fork is likely not worn out if worn at all.
Charley Fitch
