The first M1 rifle barrels to have a date (month & year) were the gas port barrels manufactured in June 1940. Note: these are very rare to find now and less than half a dozen are known to survive. Beside new production, these barrels were also used on "direct conversions" too.
Also note there is an extra circular groove cut behind the gas cylinder lock threads. do not know the purpose but once in awhile you will other mid 1940 barrels with this extra feature too.Information
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