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But I've never seen significant wear on the brass pins. And they should wear more than the steel, no?

Another factor is that the holes had to be drilled before heat treatment.
But if you have just forged a blade, you have to either
1) wait for the blade to gently cool off from the correct...

One good argument agains the 'wear and tear' theory is that the holes are often a bit crooked. If it was wear, both sides of the holes would be worn so that any irregularities would be worn away ....