[lingodog]

An interesting question. I have a number of US Flight Manuals for the type.
Tech Order 01-60FC-1 dated August 1941 for AT-6, AT-6A, AT-6B & SNJ-3
Tech Order 01-60FE-1 dated April 1944 for AT-6C, AT-6D & SNJ-5
Air Training Command 51-60-1 dated March 1948 for 'AT-6'
Tech Order 1T-6F-1 (formerly 01-60FFC-1) dated January 1952 for T-6F & SNJ-6.
So on the scant information I have it was sometime between March 1948 and January 1952