For the record I bought a Harris Cyclery FF hub. It is threaded for fixed and freewheel on both sides. The Surly Dingle screws on normally, clockwise, on the larger threads used for the freewheel. The lockring I bought, thinking I really needed it, screws onto the smaller diameter thread counter clockwise.
Note if you do not like the unique dingle dual sprocket there are no splines to insert a key to get it off. As you ride you will tighten it. I doubt there is an easy way to get it off. Can you hit a chain wip handle with a hammer?
If you doubt it, pay the $20 to Harris, try it, if it does not thread on return it/exchange it. I think they will take it back 100% unused.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.