I purchased this bright white beauty of a mug with hand-painted "Greek Key" decoration from a dealer in New Hampshire with always unexpected things at great prices; recently from him I grabbed a huge, fine, 18th Chinese export punch bowl with gorgeous staple repairs. I've done a fair bit of searching and can find no exact likeness, but turned up a couple of references, the most useful a c. 1863 Copeland (British) basic and pitcher with hand-painted decoration in black and pink (see last two photos.) This one has no clear makers marks from what I can tell, but was numbered by hand and shows a couple of impressed letters and numbers, though a bit hard to discern. I would indeed guess mid 19th c. or so, hand-painted rather than transfer decorated, with leaf pattern on the handle a perfect complement to the geometry of the border I think-- and in pretty much perfect condition.
3 1/2" t x 3 3/8" d x 4 1/2" across at handle. Excellent antique condition.