71 198 slant new head

Doug Dutra's book seems to be saying that the rocker arm shaft was changed at the same time the special shoulder bolt was introduced. According to the parts catalogs and service manuals the special bolt first appeared on the 1975 engines. What's confusing is the rocker shaft 1947614 was used from 1960 thru 1980. Maybe some of the slant six experts can explain this.