In 1956 Chevy increased the compression ratio from 7.5 to 8 to 1 and then to 8.25 to 1 in 1958 on the 235. They did this by making the combustion chamber smaller. The "848" heads were used from 1956-62. In order to maximize the performance potential of the 235, the "848" head is recommended because of these smaller chambers. Not sure if there is any real difference between the 56-57 heads and the 58-62 ones, It may just be a "marketing ploy".