Hello Christian.
The book "The Identification and Dating of Sheffield Electroplated Wares 1843-1943" by E.R. Matheau-Raven does show a M&Co Ltd mark that they attribute to R.F. Mosley & Co Ltd.
However, it does not resemble the M&Co mark posted here.
Also, I'm not convinced that the book's attribution is correct.
Have seen the M&Co in a shield mark many times, usually on pieces that look to be made in the 2nd half of the 20th century.

I have been thinking the M&Co in a shield mark might have been used by Marples & Company which became a subsidiary of Martin, Hall & Company.
However, I have not been able to confirm that.
Pat.