I think it depends on your definition of a "make" vs. "model" vs. "design". Parker certainly called the Deluxe and Custom different models than the Standard or Super, so according to Parker there were at least 4 models, even if three of those models were the same design (Mark II).
From Tony Fischers site:
QUOTE
The clips in 1956 were:
• Standard with satin Lustraloy cap and steel ridge clip
• Standard with satin Lustraloy cap and inverted steel ridge clip
• The DeLuxe with bright lustraloy cap and gold plated ridge clip
• The Costum with gold plated cap and ridge clip
• The Super with satin lustraloy cap and ridge clip with Parker 45-style imprint
• The Super with satin lustraloy cap and Parker 45-style clip
Perhaps the better question for Fiberglassworks would be - do you have more than 3 overall nib/feed designs? Another question - if there are more than 3 overall nib-feed designs, are the other designs more than one-offs or prototypes? (If ParkerCrazy said he had some additional Parker 21 designs among the prototypes he comes up with, I would not doubt it for a minute).
A more general question - how many overall models are there when you combine the Parker-named models with the 3 different designs? I think it runs like this:
Mark I,
Standard (with 2 clip variations)
Deluxe
(Fishscale)
(Demonstrator
)
Mark II,
Standard (with 2 clip variations)
Deluxe
Custom
Flighter (did Parker call it a 21 Flighter, or was it just a 21 Standard in steel?)
Super (with 2 clip variations)
(Demonstrator)
As a final note, while Richard Binder's website is a phenomenal source of information on pens and pen material, it is important to keep in mind what he says at the end of each article.
QUOTE
The information in this article is as accurate as possible, but you should not take it as absolutely authoritative.
A good approach to take to all pen references. I would say. . .
John