Below is a comparative picture of a Remington M1903 (top) stock with a M1903A3 (note the "slots") stock (the stock is mounted on a earlier Remington 1903 rifle at bottom. The "slots" were for a M1903A3 handguard ring, which held the different 03A3 handguard in place.
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Below is a picture of an earlier Remington M1903 (3,051,000 range) with the subinspection marks I mentioned - there were numerous different marking "shapes":
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