I'll quote from
Doug Smith's web page which can be linked to from the
probus page talked about in a previous post.
"The mark
XXI, the
Roman numeral for 21 or 20:1, was translated into Greek the numerals KA (=20+1) at some eastern mints. This
antoninianus of
Probus was produced at
Serdica by the fourth ( follows KA)
officina. What is unusual about this coin is that while most coins of
Serdica bore no city mark, this shows MS (
moneta Serdicae) in the
field. This is similar to the common
Cyzicus use of MC or CM. This particular
type showing
Victory and the Emperor is not listed in
RIC. A few other reverses, however, do show this
style mintmark. Recent
hoards found in the
area of the Eastern Empire are making the discovery of new
types a common occurrence."