I have read that on the first run of 15s with the spear blade main, the pen was designed to be pushed down into the handle to access the spear.
I agree that when the Spear Point Charlow with Pen Blade came out, the long pull on the Spear Point Primary blade was hidden by the pen. Two things were done to make access to the long pull easier.
1. A Swedge was put on the pile side of the pen blade, to allow the fingernail to reach behind the pen. This was the pattern that put the pen blade in front of the Primary blade, so both nail nicks were on the mark side of the knife.
With this setup, it was pointed out that once you get your finger nail into the long pull, you can leverage the pen blade to sink, by pressing with the thumb on the pen blade while lifting the Primary blade.
I then became aware that all of my knives, multiple brands of Barlows with 2 blades including Russell, Camillus, Remington, Boker, and GEC, had the "feature" of being able to press on the pen blade and it would sink deeper into the pocket. It turns out the same is true for all of the primary blades.
2. A run of Spear Point Charlows was made with the Pen blade behind the primary, and the nail nick of the pen blade was on the pile side. This completely solves the problem of access to the Primary blade long pull. But there was a majority of people that thought having the nail nicks all on the mark side was "prettier" to look at.
What followed were Charlows with Clip Point and Sheepfoot blades, both of which sit much higher out of the pocket than Spear Point blades. This pretty much eliminated the access problem to the long pull nick of the primary blade, even though the back half of the pull, toward the tang was hidden by the pen blade, you could at least get to the front edge of the long pull.
Then came the Spey bladed Charlows. The spey blade sits deeper in the pocket than any of the other styles mentioned. None of the Spey Charlows were offered with a Pen blade, instead, the two blade versions put the Spey in front, and a clip point behind, both with nail nicks on the Pile side. There was no access problem to the nail nick of the Clip blade, since the Spey blade sits so low, and the spine of the clip angles upwards.
Now back to the question of why one style of kick was chosen over another.
The kick on the right allows the blade to sink more easily into the pocket, if the spine of the blade is pushed down while the thumbnail is in the nail nick of the primary blade. However, if I press hard enough, even flat kicks can be made to press on the back spring when pressure is applied to the spine of the blade.
So I would say, that the tapered kick shown on the right in the photo above, makes it easier for the pen blade to sink into the pocket if the thumbnail is used as a lever when reaching into the nail nick of the primary blade behind the pen.
Maybe this picture will help illustrate. My thumb nail is in the primary long pull, and I am using the same thumb to squash the pen into the pocket.
Try this, pull out some of your two blade knives, and while the blades are closed, press on the spine of the pen blade. You will see that you can make the pen blade sink deeper into the pocket, if you apply pressure to the spine.
So my conclusion is that the tapered style kick on the pen blade makes it easier for someone to get the pen blade to sink more easily, in order to get the primary blade open, if leverage is used against the pen blade while the thumbnail is in the primary nail nick behind the pen.
Of course none of this pressing on the pen blade to access the primary nail nick is required if the knife is designed with the pen blade behind the primary, and with the pen blades nail nick on the pile side. That is how my Case Peanut is setup, pen behind. I favor that arrangement.
So to me, the Spear Point Charlow with pen behind, and its nail nick on the pile side is a Grail knife.
