Did further searches. Looking to Marcus Anghels page it should be the following
M7 66 >>> Monroe Plant, Michigan 1966
10 20 1 >>> Oct, 20, First shift
Full date >>> Produced: October 20, 1966, First shift
Yes matches what was said earlier. There were a few different patterns used through the years. Some of the same information and more (for other parts) is available in the article in the Library section of the site titled
Decoding Date Codes 
But I do not understand fully the process of the "rework" and the plier marks for rechroming. How does this happen?
It seems also my bumper has partly been welded in corners left and right to the number plate radii.
The bumpers are clamped, twisted, bent to get them into shape then beat with different tools. One of the more common tools and the one that left those big nasty marks is a big hammer that has a slim head with a somewhat beveled edges that is used to strike and roughly shape the bumper into the general shape while its held over or against a very big anvil. Have seen some shops that also use some pretty big/tall machines to straighten and achieved the general shape also. Also heat can be used to make the metal more workable but it makes an already, often hot shop and workers even hotter so sometimes it just easier to beat the metal harder and more often than to be very uncomfortable
Then the outer surface is ground to acceptable limits then plated with multiple base coats of metal to hid some of the small amount of work on the exterior while ignoring the back side. Of course all of this shaping and grinding removes metal making them weaker and the metal thinner in places
Here is a video that really does not cover the process but gives some great in-site to the world of repaired and rechromed chrome bumpers
