All you can do is speculate. Scorch may be right that he didn't know how to fit it, but why install such a mismatch in the first place? The simplest explanation is that the part came off another gun it had already been fitted to. Maybe the original was defective and broke, or perhaps it didn't extract properly and needed its tension set and the fellow didn't know how to do that? More likely, though, is simply that he needed the original for another gun, then he had to put something in the Springfield to be able to sell it.
If he did that filing to the part intentionally, the only thing I can think of is that the extractor may have been slightly long and the rear edge of the extractor hook cut was protruding from the breech face? That will mark cases. By filing the forward side of the slide stop notch you could get that surface to be flush with the breech face, though you would have to upset the rear edge of the firing pin stop to pull it back and have a tight fit. It would be an awkward approach, when you could just clean up the fit at the extractor hook itself along with doing the usual tuning contouring and clean-up.
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