A mystery.
My best guess is that the M is a sigma, i.e, this is a
Roman standard half ounce.
On the other hand there are
plenty of
Roman period lead half ounces in the Pera Museum, but they are all marked with an S (although ounces are marked with an A), and they are quite different in
style to this object.
Perhaps this is some unofficial local
weight.
As to the Lydian
standard, I don't know of any
weights that can be definitely attributed to
Lydia. although one of Tekin's standards of the Aegean region is a mina of c. 420 gm, which is close to the
standard of the
electrum coins, which is (I presume) a mina of 30 x 14.15 = 425 gm.
Ross G.