There are THREE pins which go onto the SMLE to lock the rear sight in place.
The TINY one goes in the end of the Axis Pin on which the Leaf (arm) of the sight rotates. It has a very small hole in the end of the Axis Pin. These are right out in the open and are obvious.
I suspect the one you are worried about is the pin for the Bed. The Rear Sight Bed surrounds the Barrel of the rifle and has the remainder of the Rear Sight assembly mounted on its top. These parts include the Spring, the Axis Pin, the Leaf and all the rest. The Bed itself is secured to the Barrel of the rifle by a Cross-pin passing through the Bed about two-thirds of the way breechward from the front end of the cylindrical portion of the Bed.
The Barrel is not DRILLED for this Pin because the pin does not pass THROUGH the Barrel; rather, it GRAZES the UPPER SURFACE of the barrel. If you have the Bed in the proper position, there should be few problems. If you slide the Bed forward, you should be able to SEE the graze for the pin quite easily.
Note that the Screw for the Spring of the Rear Sight Leaf also passes through the Bed, vertically in this case, and may be used to position the Bed for the installation of the Cross-pin. You should be able to SEE where this screw hits the barrel if you have everything apart.
Once you know where everything goes, it's just line everything up and put it together.
Hope this helps.
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