273 rocker arms

/6's are not a comparable example: all solid lifters and adjustable rockers 'til the last few years. So no need for the groove in the shaft. But they had grooves in the rockers for oil distribution from the single oil hole in the shaft for many years for both types, and when they went to the hydraulics in the last few years, the rocker grooves changed a bit.
Hydraulic vs. Mechanical cam valve train differences - Slant Six Forum