Johnson Bros Venetian Plate

This platter approx. 10x16 was from my mother-in-laws house it has several particles which looks like cardboard on the underside that are baked into the plaster. It also has a stamp impressed into the plaster and not colored so it is difficult to read and can not tell if it is a 2 or 3 as the first number it says MNo.277074. There is also a random M and C in blue. I believe it may have been her mothers or grandmothers, which would put it in the mid to late 1800's possibly early 1900. Her family is German her husband was from Prussia and was in the Navy in WWII.