EAN-6564-B1 was the ENGINEERING PN for the 1954 to mid 1956 rocker arm (1.43), when FORD changed the ratio in mid-56 production (1.54). FORD then went back to 1.43 ration with the 1958 model run (FYB HP was over). The SERVICE PN was upgraded to C0AZ 6564-A. 58/59 rocker arms had the PN/CASTING ID used by FORD during 58/59 consisting of seven numeric characters.
Not too sure when FORD dropped 1.54 service replacement rocker arms but this is why you will see assemblies with mixed ratios.
W (BY GOD) V - EASTERN PANHANDLE