I can only provide the info from the parts book...
Section 0.333
Valve Rocker Arms are the same for all 6-cylinder engines 1950-58.
Valve Rocker Arms are the same for all 6-cylinder engines 1959-62.
Per Section 0.269 the 848 Head was used from 1955-62 which spans both different sets of rocker arms...someone else will need to answer why they used different rockers on the same head.
Mike B
