Would have been better with a pic of the item and the marking !! But the most likely scenario is that it was a gift to someone in that year and they used that little home engraver to put the year and M for whoever gave it !! Those things are not the easiest to use and I would have done the same thing !! Just put the M for the first letter of the name instead of trying to do the entire name !! Could have also stood for Mother and 1964 the year of her passing !! Not all of us have great memories for dates !! I don`t !!
Thanks for the reply. Sorry for no pics, I don't have a digital camera, or probably more importantly, the necessary expertise. One of these days I should get one and learn. I know just a description can be somewhat confusing and hard for someone to picture what is being described.
I also had thought about the scenario you mentioned and it seems very likely. The "1964" and the "m" just has the appearance of being done by someone at home with an engraving tool and not by a company. As I had mentioned, there were too many obvious differences; one "m" is taller than the other and one "6" has a bigger loop in it, plus other differences.
I did learn in researching on the Internet the name "Rockford Silverplate Co" dates from 1882 to 1925, when the name then changed to Sheets-Rockford.
Thanks again.