My guess would be that many of the parts are machined in house, not off the shelf pre-fabbed pieces jigsawed in to work with the design of each knife.
So, unless the manufacturer plans for people to lose parts and keeps extras on hand, you may have no other options for getting replacements
I think sometimes we take for granted how unique even production knives may need to be in order to get perfect spacing, tight tolerances, and good pivot action.
It's going to be easier to make it themselves how they need it, specific to each knife, rather than build their design around such a small piece.
I'm not familiar with the inner workings of that particular knife, but generally speaking, a stop pin has to be the right diameter to stop blade movement when open and closed, and the length will be different depending on blade width, spacing between blade and handles, and how deep they sit into the handles. And depending on the manufacturer they will be different diameter, they can be straight cylinders, they can have shoulders milled into them to be thicker in the center, I've even had ones i swear are slightly tapered on the ends from the way they seem to stick in their holes
In other words, sometimes a pin is not a pin is not a pin