This is a later version of the same lens described in the previous page (model II). This lens does not have the automatic/manual (A/M) switch for the aperture mechanism (just like the SX series lenses). This fact was first mentioned to me by a member of the excellent M42 forum.
Interestingly, the writing on the front ring of the lens and the aperture ring look exactly like the SX series lenses. The lens is also a little bit lighter compared to the model I version. For a comparison between model I and model II of this lens please see below.
I tested this lens recently, and the optical quality is superb and already excellent wide open.