The 19 NRA's were never D&T'd for scope mounts from the factory - top or side (very few factory rifles made before the 1950's were) - and the original rear sight mounted using two screws on top behind the ejection port so you're looking at a gunsmith's handiwork. As such the screws could be any size, but I would suspect they would be either 6-48 or 8-40 which are the two standards for scope mounts and pretty much have been forever.
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