OK - so this is a correct and interesting unusual Parker item. I am still a little perplexed by what looks to me like different chasing on the cap and barrel. But that is not really important to the discussion of the clip.
Now that I have a name for the clip, I have some "quick and dirty" Google results
There is a patent date on the clip in one item on ebay in the past
I just started looking at that patent date and found some cute things for clipping pens to pencils, but not for this clip yet.
I am not the definitive expert. However, the crack at the top of the clip usually indicates that the original clip was broken off. The current clip (in my opinion) has a homemade appearance to it.
There are Parker pens with VV clips, but those have a transverse pin to hold them, and this does not appear to have that type of hole. Then again, I am not familiar with a long slit like this for any type of clip that I can think of. So that cut in the cap is a bit of a mystery to me.
Furthermore, the chasing pattern of the cap does not match the barrel. In my opinion that cap is old, and a good fit, but not original.
The earlier Swan lever filling pens, of which this is an example, are from the early 1920s. This might even have "SF2" stamped on it somewhere. There do not seem to be further detailed model numbers. Some were stamped "posting" on the barrel, or like this one, "stenograph" to indicate the original nib type. Most had no such notation.