Sling eyes were an option ( they did not swivel ) on the older Winchesters. They were installed just as you describe on the fore arm band with a rivet. On a 50's gun probably not original, but never say never with Winchester. As stated above they were not standard but could be ordered on the gun OR ordered later from the factory and added. The only way to know if they were original is with a letter from Cody Museum and that is available on guns below serial numbers around 354,000 only.
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