Just for anyone's future reference, I sent both freewheels to the good folks at Yellow Jersey, and they hooked me right up. They said my extra 20T would not fit in the "second position", so they found me a N.O.S 19T that would!
They said that the cog threads are the same on both english and french threaded freewheels. I neglected to mention that both of these are N.O.S.
I now have a 4-speed freewheel on my hub that has the exact gearing I wanted: 14-19-24-28. Now, if I can just get the Cyclo Mk7 to accept the 28T, I'll be fine. I hope I don't have to wait for a Mk8, cause they are hard to find.