"It looks like the front sight base might be pinned through an original (half-moon) sight base.
But that doesn't make sense either if the barrel has been shortened.
So how was this done?"

Well, the barrel could have been cut and the original half moon front sight silver soldered to the barrel, leaving the rear notch alone. Then later on, the owner decided he wants adjustable sights and had the rear sight and front sight added.

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