First interpretation: weight.
A rough square, stamped B (?) on one side; on the other traces of decoration.
Turkish (?)
ADDENDA, 20 July 1948: Clipped Turkish coin. Turned over to coin department with ... 1 April 1936 ... ΚΚ 391
Half of an unglazed lamp of coarse, highly micaceous clay like that of a household pot.
Bottom nearly flat; sides flaring, then concave in profile. Flat rim.
Probably not Attic.
Type 16 variant of Agora ... 1 April 1936 ... ΚΚ 411