The original version of the Fairbairn-Sykes knife had a ricasso to let the user wrap his index finger around the quillon for a secure grip. Much like the ricasso on that Camillus version you’re showing, Horn Dog. The feature was swiftly dropped during WWII for the sake of faster wartime production.

I’m willing to assume that Fairbairn knew what he was about, back in Shanghai. Anyone who has survived hundreds of violent street fights, of the sort that left many policemen dead, knows more about real world combat than I do. I pay attention when he talks about how to design and use a knife. I always thought it a shame that the ricasso was dropped from the S-F dagger.