That "big nut" is actually the top of a tapered pin that keeps the rear jaw in a fixed position. If you can get it out, then the back jaw swivel either side. Getting the pin out can be either relatively easy, or extremely difficult. My guess is yours falls in the extremely category.
If you remove the slide then you will find that you will be able to see the other end of the tapered pin. Drive the pin out that holds the main nut in place and remove the nut. Depending on how much "crud" is in the dovetail slot, you may have to drive it out. Be careful as the nuts are cast. Once the nut is out of the way you'll be able to get a better look at the bottom of the pin.You can try tapping it with a hammer, but you can't get a very good whack at it due to the limited room.
A better way is to get one of the little machinists jacks and push the pin out from the bottom. Obviously this is easier said than done. Be very careful that you don't break the sides of the dovetail that holds the nut.
While you're getting all this setup, make shire to spray around the top of the pin with PB Blaster or Kroil.
Put as much pressure as you can on the bottom of the pin. Use your favorite hammer, and tap the casting around the pin. This may be enough and the pin will come out. Keep in mind that, depending on how much pressure your jack is exerting on the bottom of the pin, that when it lets go, it is going to go somewhere in a hurry. Don't be looking down on it when using your hammer.
If this doesn't do it you may have to resort to your torch to warm up the area around the pin. This should expand the hole enough to free the pin. Try to keep the heat away from the pin.
As Lou pointed out, there is no such thing as a bad Prentiss vise!
Good luck, Mike
