Based on the vintage, and assuming a steel OEM quality headset, I'd expect them to be 5/32" balls. More modern or better quality headsets would use 3/16". Balls are cheap enough, that I'd make a calculated guess for one or the other, or splurge for a small package of each.
You can't mistakingly use the wrong size balls because it'll be obvious. If you're not sure load the top race and test it against the upper head cup/cone without the fork to see if the overlap looks and feels right.
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