Yes the Rem 513T was a WW11 trainer. "O" would be too early. Are you sure that "O" isn't a "K"? That would be 1941. Acorrding to Batha in his US Martial .22RF book delivers started in 1941 and ending in 1944. For a total of 69,952 rifles.
Therefore all rifles should carry a barrel code of K-L-MM-NN. Let us know when you look again.
My 513-T is about 3900 less than yours.