I once owned a Suntour Perfect freewheel that had smaller threads in the outside cog position, allowing it to take a 13-tooth small cog. They're not very common, but there are some around, I think. What I can't recall, and haven't been able to find out, is whether these freewheels are designed for a the standard six-speed 126mm OLD, or for the 120mm six-speed ultra variant.

What I'm asking, I guess, is this: Did Suntour (or anyone else, for that matter) make a 120-spaced 6-speed freewheel that would accept a 13-tooth cog? At least one inquiring mind wants to know.