Change appears (for the ribs and untrimmed upper water pump side bolt holes) to have taken place "about" mid 67 production. Since the engines were not supplied in a particular order the change will likely be a "soft" one spanning over a number of weeks at least.
All is going to relate back to the timing cover itself and the casting date which is not easy to collect from an assembled car.
Still narrowing it down starting with San Jose since there are more examples out there in that group. Difficult on any cars to determine if the timing covers or engines have been changed out. Currently the last two ribbed examples are from a restored car and a rebodied Shelby so those are not solid findings. Currently I think that Duce's is the latest (engine assembled 6L25) is the latest solid number but have a mid Dec 66 built example that is a possibility.
Earliest (still looking) smooth examples are pretty solid. Five unrestored examples within a couple of weeks span
Will see after we get more examples and data
