I only have the 75 version of the MPC which only list the part number for the Powered by Ford valve covers, but under PCV hose it lists C7ZZ-6A664-A for 67 Fairlane GT and Mustang 4/B before 10/3/66. So there is a rough change over date.
But that date does not seem to be supported by examples we've seen so far. The MPCs are not without some confusing information at time - example my copy shows 66 Mustangs equipped with 390 equipment listed
Closest page I have is a 4/68 version showing the same right side valve cover (early - late starting 67 production) with a C6OZ6582-A preceding it and shown with an em/te hole
The left side valve cover is listed as being changed with the Level 8 change that year from C6OZ-6582-B TO C7OZ-6582-C. Given the other footnotes on other pages it appears that the change to L8 was the 10/3/66 date.
Notes also indicate that for other (non-chrome equipped) applications the left valve cover also changed but the right hand one stayed the same before and after the L8 introduction. This change in the valve cover appears to correspond with a change in the PCV grommet.
Got to ask did Ford use "left" and "right" in this application backwards from the left and/or right side of the vehicle? Hate using left and right when describing things on cars- always taught passenger or drivers cars during extrication course I taught to address the possible miscommunication
Unfortunately no nice detailed pictures for reference.
Still have the issue with the information maybe not matching with how the car may have been produced. Will try and find time to look through the TSB articles to see if there is a comment or note there to help the discussion