The Security and Speed Six are the same gun, except that the Security has adjustable sights while the Speed has fixed. While they could be considered K-frame-size revolvers if compared with S&W revolvers, the GP-100 is the equivalent of the L-frame S&W guns like the 686 -- slightly larger frame, slightly beefed-up in critical areas to make them stronger and even more durable than the already-hardy Security Six and Speed Six. Without going in to minor action differences, those are the essential the differences between them.