Over the years much has been made of the differences in top nuts and the marks impressed into them. According to Robt. Wilkinson Latham in his excellent book on Wilkinson kniat least some of the brass nuts were made by an automotive parts mfgr. That makes complete sense to me. They are not unlike the nuts on the ends of sparkplugs.

The impressions in the soft brass nut were created by clamping the nut in a vise for tightening the assembly. To assemble a F~S one would fit the guard over the tang, slide on the handle, and screw down the top nut. Then, clamping the nut in the vise, you would use a spanner of some sort to tighten the blade into the nut. It is the crosshatching on the vise jaws  that left the marks. Once it was tight the excess tang could be ground off leaving enough to peen over, securing the nut.

Not all nuts have the marks indicating that they were assembled differently or that a jig of sorts was used to hold the nut without damaging it.

Typical recessed top nut on fat wood handled knives

Top nut used on wood handled knives of the third pattern style. The  tang is carefully peened over to secure the nut without cracking the wood.

Indian top nut with blind hole. Tang does not pass through.