Fig. 40.1
The camshafts in Fig. 40.1 show the difference between the 283-CID and later camshafts "A" and the early camshafts used in the 265-CID block "B" has a slot shown by the ARROW to supply oil to the lifters, valves etc. The back cam bearing has two holes, as the cam rotates the slot connects these holes to supply squirts of oil to the lifter galleries. With the introduction of the 283 a modification was made around the rear cam bearing bore. An annular groove was machined into the rear cam bearing bore to supply a constant supply of oil to the lifters.
See 265-CID Cam Bearing for more information.