It's likely just a foreign object on the plate at printing, if it's "defined", it's hard to tell if its a smear otherwise. Have seen these a lot in many types of engraved plates. Doesn't look like a crack to me, too straight a line.
Sorry, on the second point, there's no information in Scott because that type of anomaly isn't significant enough. In generally it falls into the category of "Errors, Freaks and Oddities", where this is a freak. It's repeatable and frequent but usually not 'permanent' in its variation.