Hi Dwight - Everyone here has been most helpful, but I still think you should call Lily Renzetti. From your description, the screw you need does not appear in Norm's photo. It is part #4 in the schematic for the Traveler I've included for future use for you.
The reason you want to use a nylon screw is that a steel screw will deform the shaft of the rotary head over time/use, making disassembly difficult or impossible. This vise also has a warranty, and whether or not it applies to a used vise is entirely up to Renzetti. Use of a non-standard screw, and the damage it could cause wouldn't help your case if factory service is required down the road.
This particular screw doesn't require much force to do it's job - all it does is provide additional friction to keep the vise's rotary shaft from spinning freely in the vise head. A nylon screw from the hardware store would work just fine for this purpose.
Part of my job involves fabrication of parts, so I'm frequently in a machine shop, both at work and at home. I have a lifetime supply of those nylon socket head cap screws a short walk from where I sit. If Lily doesn't take care of you, send me a PM and I'll gladly send you a half dozen screws of the correct size. The cost? Merry Christmas!
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