I'd guess overpolishing in that specific area for some reason? Perhaps there was a forging flaw on both sides that had to be removed. There honestly, typically, isn't much value you can expect to glean from idiosyncrasies in antiques. 9/10 times the answer to "why does this (idiosyncrasy) exist?" is "it just does"; it's hard to say much more since we can't personally ask the artisans responsible for producing the item.
Again though, I'd imagine it has something to do with excessive polishing/grinding in that particular spot.
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