For the potato salad: Put the potatoes in a medium pot and cover water by 1 inch. Bring to a boil, season with salt and cook until tender, 12 to 15 minutes. Drain the potatoes and return them to the hot pot.
While the potatoes are cooking, add the bacon to a medium skillet and cook over medium heat until crisp. With a slotted spoon, remove the bacon to a paper-towel-lined plate. Pour off all but 2 tablespoons bacon fat from the skillet. Add the onions and garlic to the bacon fat and cook for a few minutes. Add the mustard and coriander seeds and cook for another 3 minutes. Add the apples and cook for 3 more minutes. Sprinkle in the brown sugar, add the stock and bring to a simmer. Cook for a couple minutes, stirring to dissolve the sugar.
In a small bowl, whisk together the cider vinegar and grainy mustard, and then slowly whisk in the canola oil.
Add the onion-apple mixture and the vinaigrette to the hot pot of drained potatoes. Set aside until cool to allow the potatoes to absorb all the flavors.
Season the potato salad with salt and pepper, then add the crisp bacon and the celery and parsley. Toss well.
For the sausage: In a large bowl, combine the pork shoulder, short rib, fatback, parsley, salt, red wine vinegar, sugar, celery seed, granulated garlic, granulated onion, nutmeg, caraway, coriander, dry mustard, paprika, black pepper and some salt. Run the mixture through a meat grinder fitted with the die with the largest holes (see Cook's Note). Repeat so that all of the meat goes through the grinder twice. Mix in the Swiss cheese and up to 1/4 cup cold water as needed.
Attach the sausage-stuffing attachment to the meat grinder, keeping the grinder blade and die in place. Place a generous amount of the prepared sausage mixture in the grinder pan. Give the casings a slight bit of resistance by holding onto the sausage stuffer with your last three fingers and letting the casing slip through your thumb and forefinger. Turn the machine on and fill the casings, being careful to avoid any air bubbles.
Twist the casings to create 5-inch sausages that are even in size. Tie each sausage with butcher's twine to keep them nicely sealed. Prick the sausages all over with a pin or skewer to vent steam during cooking.
Place the sausage links in boiling water and parboil for 10 to 15 minutes. Slice the sausages in half lengthwise and saute until browned in a preheated saute pan slicked with olive oil.
For the thousand island sauce: Combine the sour cream, ketchup, relish, Worcestershire and some salt and pepper in a bowl. Stir to blend.
To assemble the sandwich: Spread a pretzel bun, top and bottom, with thousand island sauce and then with brown mustard. Add sauerkraut and sausage halves to the bottom bun and cover with the bun top.
For best results, freeze the meat grinder overnight before making the sausage.