I am, HJ, bottom of page 20 of the catalog in question. (The gun at top of same page, without screw-grip top extension, obviously is not a screw grip, and is not priced as one.)
The screw-grip gun at bottom of page has same pin placement as Evans and does not show the top extension, as the gun is closed. Obviously the example in catalog has one.
There are two basic types of screw-grip top extensions; the doll's head variant and a straight, stepped extension. The ones I've owned had doll's head.
Am I missing something you are seeing?
