Patrick Neal comments below that he volunteers at the Fort Vancouver National Historic Site Carpenters' Shop where there are two Nicholson benches built from Nicholson's instructions. I spent some time tonight searching for photos and found the following panorama, which is copyrighted so I'll just link to it here. Carpenter's Shop Panorama Read about the photographer here: Don Bain. The picture is quite unusual in that you can click on it with your mouse and look around the shop.
Notice that the benches only have the front sideboard, which is authentic. However, notice the vise on the bench to the left against the wall. Do you see what's on the guide? Looks to me that it functions like a horizontal leg vise with holes and a peg in the guide. Is it the functional equivalent of a twin screw vise?
I'm definitely going to go for a visit soon, take some detailed pictures to post here and try to learn more about their historical research. Thank you Patrick.