Looking through a number of illustrations, it seems as though the foresight BLOCK was integral with the barrel, but had a slot cut into it for the foresight blade. Was the hole and pin added afterwards or was it a later manufacturing add-on? Looking at a number of these, it seems the hole for the pin is in exactly the same place on all - in the center of the block, just below the round-over - which suggests it was 'design'.
I suspect the original blade was BRASS, as many were at this time. It looks as if it was a blade originally, but could easily be replaced with a bead or even a ball in later life. If it fitted into that slot, it would need to be secure, otherwise the recoil would have dislodged it. Some appear to have no pin at all and some show that pin. The brass blade also seems to be dovetailed onto an iron one to hold the pin on some of them.
Brass wears over time, altering the zero. However, if you replace that brass one with an iron one, it will need securing. It also seems as if the receiving slot was deepened on a few for that purpose too.
(photos from College Hill Arsenal (sold) 1 x rifle, 2 x carbine)
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One of the photos from
bobinwmass above suggests that the foresight BLOCK was reshaped into a 'bead' sight.