[Mr70]

The BB front one,-Left one..aka "Hockey stick" design,Part 3940933,will not work on vehicles with Air Conditioning,as the compressor/bracket gets in the way.
They used they the later issued #3981061 "Over the center of Valve cover" design for that,and continued with it well into the 1970's.
The 1970 Chevelle Assembly manual calls out the Left side revision on 9/12/69,but I can tell you that it didn't happen on all Chevelle/Elco's that fast and at all 6 solid lifter plants at the same time.Chevrolet used up the earlier issued "Hockey stick" design up to around January of 1970.
This is my own opinion based on talking to original owners,seeing their vehicles,Chevrolet archival photos and Car magazines road test articles on these vehicles from back then.