In order to learn about this tankard, I researched the internet and asked various experts. Here are there answers and descriptions:

   "Old Sheffeld Plate (aka fused plate) was invented in 1742 as a sandwich of silver and copper fused together. OSP tankards would show a seam running down the tankard from the construction, and the upper edges would either have silver wire soldered over the raw edge to conceal the copper, or be rolled. Tankards made of OSP often had a wooden base, and might have a disc of sterling let into the side to hold a monogram, crest or arms. First period OSP would have silver on one side, and the interior would be tinned, but material rapidly developed to a sandwich of a sheet of copper with a layer of silver on both sides. Your tankard looks about 1770 and if it is OSP, nice find - there are enthusiastic collectors." As you can see, there's a seam under the handle, and the silver and copper are clearly sandwiched in the photo of the base's edge. This doesn't have the sterling or wood.

As to why there are no marks at all, "there was about a decade (1773 to 1784) when it was illegal in England to apply any marks to fused plated wares. A huge amount of OSP was produced during that time and is often misidentified even these days as either from China or India." 

Size is 5 7/8" high x 4 3/4" across the base, 3 3/4" across the top. There doesn't appear to be no copper showing through, but it's difficult to tell with the heavy patina. Possibly on the handle? See Condition notes.