It's a "Standard" model. The Mark 1 was the same but with adjustable sights. Later numbers represented changes to the platform.It is a very early standard model with fixed sights. Looks to be a 1968 production year according to Serial number. It's fairly early as the Left hand grip has the descended grip screw, the lack of scallops on the rear of the frame by the charging bolt. This is known as the A-54 frame. It is the most iconic handguns ever.