The standard pin has a larger head complete with an Allen screw for locking in place.
There's also an Allen screw tension adjustment to regulate how tightly the pistol is held.
Both blued and stainless versions are offered with an Allen screw in the head of the pin that can be tightened against the bottom of the barrel to prevent movement under recoil.
One of the really nice things about the little rifle is that changing barrels is very easy - simply remove two Allen screws.
All you have to do is loosen - and then tighten - two Allen screws to change barrels.
Origin
1930s: from the name of the manufacturer (see Allen key).