This image is the one in the front of every copy of the First Folio of Shakespeare plays published in 1623. It was cut onto the printing plate by Martin Droeshout. his name is at the bottom of every print of it, though perhaps a little small to read on this picture.

As can be seen, the letters M, n and Y are across the chest. The strange collar around the neck is simply a large letter D.
There is a rebus for Moon, in the form of two crescent moons under the eyes.
To add to these are two representations of, day. One is the reflected sunlight on the forehead, the second is the shadow at the side of the face.
M n D Y in letters, then supported by the Moon rebus and the Day rebus.
Moonday > Monday > Munday.
The brass printing plate was altered a few times during its several print runs. These alterations can all be viewed at the Folger Shakespeare Library, online. They have provided a series of of images of the various changes through the print runs.
The first alteration was made almost as soon as the very first print run commenced. We know this because there are circa 200 extant First Folios, and only four do not have the shadow to the side of the head. The print run was stopped almost as soon as it was started, and the brass printing plate removed so that it could be altered. Droeshout didn’t do the obvious for his commissioners though, and remove the shiny patch from the forehead. He did what they asked him to do, add a shadow. That was the only alteration, He added a shadow to the side of the head. It only makes sense when you realise that it was added to further emphasise ‘day’.
There is more to this, I will reveal it next time. Or, buy my little book, “Shakespeare’s Mentor” from Amazon or Lulu books online to read everything I know about the real London-Shakespeare.