Afraid I can't tell you what the thread in the sling bar is either even after checking mine because I was curious. It appears to be a proprietary thread as it doesn't match either anything either SAE or Metric. The bolt in mine measures .286" x ca. 30 TPI. It is between 28 and 32 TPI, my thread gauge is pretty standard. I expected it to be 1/4 x 28 TPI, but it isn't even close. My rifle is ca. 1959, no reason they would have used a metric thread at that time!

Sorry I couldn't help, thought this would be easy!

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Gunsmithing since 1961