Interesting topic.

I have the same problem with the old style 600 hubs that have a screw on last cog.

Now as others mention just buy a 5/6 speed pignon remove the cogs and handfile the biggest spline to fit.

No problem there and use the spacers.

So so good,but what if one doesn't have the last screw on cog to secure the rest.

Is there an alternative to get.