https://rpc.ashmus.ox.ac.uk/countermark/560 

GIC 439

star, incuse, with six rays

incuse 

3.5 mm ray from central point 

Cyzicus (Mysia)

possibly not a cmk, since it invites comparison with the Christian graffiti, including a chi-rho monogram, on a coin from Caracalla from the same mint (M. J. Price Coins and the Bible, 1975, p.28)