This problem was cured with the newer breech bolt of the 'HS' Model 39's and guns made after that. The higher pressures developed by the high speed ammunition had a tendency to crack the breech bolt of the older, non HS guns. The HS did refer to High Speed ammunition and prior to the guns with the HS prefix, it was not a good idea to use high speed ammunition in guns made prior to 1935. The Model 39 was made from 1921 to 1939 and only Model 39's made from 1935 o 1939 will have the 'HS' serial number prefix. The reference to the 'HS' is for the Marlin Model 39, not the 39-A.
