Does anyone know of a source for a custom magazine tube for a 410 Ga Remington model 870 shotgun? My customer wants to extend the tube to the end of an 18 ½” barrel.

A couple of thoughts. The Marlin Cowboy 45-70 has a 26" barrel and full magazine tube. Possibly that could be adapted. You might try Wilson Combat as they have made some very long 12 ga. tubes for competition shooters like Jerry Micaluk (sp?), appearing to be about 2 ft. longer than the barrel.
I think that any way you do it, it is probably going to be big bucks to get it right, possibly more than the gun is worth. Let us know if you find anything.

Take the nut to a shop, have the thread duplicated and install that on a tube.
I'm my experience it's going to cost more then the gun is worth.

Edit: you'll also need a longer spring.