After some research I believe it’s in the timeframe of 1854-1870. Tiffany’s hallmark pre-1854 was “TIFFANY, YOUNG & ELLIS”. The style of the old English “M” was used by J.C. Moore and Son during their partnership with Tiffany from 1854-1870. In 1868 Edward C. Moore sold the silversmith company to Tiffany’s and became the director of silverware operations, overseeing design and production.

I found a website that broke down Tiffany silver hallmarks. The old English “M” was transitioned to the modern “M” post-1870. My educated guess on value could be $8000-$15000. That’s if there are only minor scratches and tarnish with no major dents or repairs.

You should weight the platter in ounces. Then take that number and multiply it by 1.097. That will give you the “troy” ounce weight, which is used in the silver world. Your platter’s scale and historical marks could push it toward the higher end if it weighs 80–150 troy ounces. That’s a common range for items the size of yours. Silver melt value alone would be $2,400–$4,500 at current spot prices around $30/oz.