This is weird, looks like my first post this morning disappeared.

650 rpm was the idle speed for 4 bbl manual and automatic, and 2 bbl manual 383. 2 bbl automatic was 600 rpm. All '68 383s were CAP equiped
Non-CAP 1967 383s all had idle speeds spec of 550 rpm. (that's with A/C on if so equiped).

The reason for the higher idle speed was that CAP traded some power and efficiency at idle for big reductions in CO and HC (but worse NOx).
But the loss of even a little power at idle has a noticble impact so higher rpm was needed to keep them runnin smooth and handle the loads when A/C was on or the car put in gear.