suggest you go to AACA forum and query diode cutouts. An example below link

http://forums.aaca.org/f120/three-brush-generators-170498.html

If you're willing to stay with a vintage-type cutout, this E-Bay seller has some for early Chevy that would be negative ground.

Park, would a model A Ford work. Cheap & plentiful. Positive GRD. They can be ordered with a Diode for reliability. I don't know how Ford cutout will mount on a Hudson generator of that vintage, or how the negative ground would affect a Ford part. Except for Cadillac, all GM cars had Negative ground before WWII.

But Robert says his Hudson is negative ground.

I saw that. How would that affect the operation. Would the voltage jump from the points ( in the cutout) backwards, or would the cutout totally not function. I had experience with a positive grd voltage regulator in my 1950 Cadillac. Kanter sold me one from a 1941, Positive ground. If the cutout points are designed for either, that is a pos/neg regulator, the regulator will work. If not then the contacts will burn out at a faster rate. So I was told.

Ron, a cutout can certainly be made to be insensitive to polarity, but most are not. There's a secondary winding in the cutout that helps ensure the contacts will pop open if the generator voltage drops below battery voltage and current starts flowing to the generator instead of from it. This arrangement won't work properly if the polarity is reversed.

Park, I presented this to the Model A guys on the Ford Barn. They all were in agreement that the NON- Diode type will work. Many Ford guys add polarity sensitive radios and the such, to their cars and no thing is needed( except the amp wires reversed) when they Ground the negative. But, probably work will be needed to make a modern Diode cutout work. Walt Bratton says the diode type is only for positive GRD as manufactured.

Thanks everyone. It's much clearer now,I will let you know the end of the story when it happens.

Any 6 volt cut-out will work, they are not polarity sensitive. However if they have been converted to using a diode than the polarity does matter.
Probably confusion because of the necessity to use only positive ground on all voltage regulators used on Hudsons, up to 1955, as these were polarity sensitive. If used on negative ground they eventually burn out the voltage coil points.

Geoff, I know that the cutout element in later regulators does have a reverse-current winding to help ensure that it opens at low generator voltages. Based on some seemingly expert chatter on another forum, I came away with the assumption that the early cutouts did also have a reverse current winding in them. Is that true? (Thinking it through, I conclude that even if they did have, it wouldn't make the unit polarity-sensitive. Kind of like the DC motor situation, where reversing the polarity doesn't bother it, since the flow in both the armature and the field is reversed, so their relationship is preserved).

All cut-outs, including the ones in voltage regulators, work the same way - there are two conjointly wound coils, a very fine one to ground, and a heavy one to the points. When the generator starts charging the current goes to ground through the fine pull-in coil, (an elector magnet) which closes the points. Once the points are closed the generator current goes to the battery through the points, and this also holds the points closed. Once the engine slows down and the generator stops charging, current flows in the reverse direction, back to the generator, which reverses the polarity of the magnetic field, which loses it's strength and causes the spring to open the points. If the pull-in coil is open circuited the points will not close, but will stay closed if you manually push the armature down., and will automatically open again once charging stops.