The Missouri was made from 1991 thru 1994 so I found an old Dixie Catalog (1995) and checked out the nipple size.
Although it didn't show this model, it did show that the CVA guns made back then use a 6mm X 1mm threaded nipple.
Track of the Wolf's new catalog says the same thing. The TOTW part number is #CVA-A (for Ampco material) or CVA-S for stainless.
Either will work and the price is the same.
The "screw that goes beside the nipple" is probably what some folks call a "clean out screw".
If this is small screw goes into the side of the drum or breech snail it can be replaced with about any screw with the same threads.
They are probably metric threads so the best bet would be to remove the barrel from the gun and take it to a Ace or similar hardware store or a Auto Parts store that carries metric fasteners.
They will be able to match it up with one of their screws.
If it is too long, use a hack saw to cut off a length about 1/8 inch long, apply some thread locking compound to it and screw it in.
It does not have to be removed to clean the gun.
Let us know what you end up doing.
