This is a large (33-32 mm) year 13 (XIII = 539/540) 20-nummia piece of Justinian (527-565). "K is the numeral for "20" in Greek, so the large K states the denomination. The mint is Cyzicus, which is also spelled Kyzicos, and its mintmark on the 40-nummia piece is KYZ. But here, with a large K already there, they decided to complete the mintmark with "YZ" smaller below. That's a cute way to give the mintmark! Sear 208, year 13. 33-32 mm. 9.58 grams. Show us an interesting mintmark!