The "848" cylinder head is a high compression head that was used between 1956 and 1963. It is most likely the most commonly available head for the 18-bolt-head 235 engines produced from 1953-1963.
Between 1955 and 1956, the compression of the 235 engine increased from 7.5 to 8.0; and, the compression increased to 8.25 in 1958.
The compression-difference in the 1956 848 head supposedly has to do with a smaller "combustion chamber". Pre '68 Dave will mostly explain this, and explain the 1958 and later increase in compression. The Shop Manual describes a 1958 change in valve to head angles, and the use of heavy-truck-like valve rotators, and heavier valve springs.
