So, I've discovered that a mug like this is properly called a "cann", and its shape is descriped as "tulip". I was able to find one that's reasonably similar in shape and style, said to date from the mid-1700's, although it's a pint cann and so half the size of the one we're discussing; you can see it HERE.
David, did you ever look down inside it for a pot touch/mark? The one you found on ebay has a pot touch. A pot mark would be inside on the bottom. --- Susan
Hi, Susan: Yes, I had a good look. There's something on the inside, on the bottom, that MAY be a touchmark, but it's so faint I can't tell for sure. It may just be an irregularity in the surface. I'm not too worried about it, though, as I've read in more than one place that it's common for pewter mugs of this era to be unmarked. Dave