I don't believe that the brass rotor stopped throughout the model 99 line during the same year. According to D.P. Murrey's book "The Ninety-Nine" the steel rotor relaced the brass rotor begining with the Model 99-E in about 1960 and serial number 1,000,000. I also know that my 99-M is from the mid to late 60's and it has a steel rotor also. Also that some 99-E's made in 1981 had a brass coloured steel rotor. I hope this helps. I'm sure there is someone else out there who knows far more about 99's then I do who may have more precise info...