The "M" would be the town mark for Mexico, and the "zig-zag" line would be for the Assay Master to test the silver, and then there should be two sets of marks, the "Maker's Mark" and the "Assay Master's Mark" . I don't have enough experience here to say if the "J.R." is the Maker's Mark or the Assay Master's Mark . The Assay Masters have their last names almost always spelled out, so I am inclined to think that "J.R." is the Maker's Mark . I wonder if this plate was produced at some point between 1900 and 1948, and then the hallmarks were made to be similar to the older "Spanish Colonial" hallmarks, kind of just to make it look like the "older style" of hallmarks . The plate does look machine-made, but the general style looks more like 19th century, so maybe this is a 20th century Mexico plate which is imitating the older style.
But again, there are experts here with much more experience with silver from "Mexico" and the general category of "Spanish Colonial Silver."