If the lead piece is fastened underneath then it comes in water and buoyant force acts on it.

Let the mass of lead piece be m.

At equilibrium : gravitational force = buoyant force

(200+m)g = (200/0.8)g + (m/11.3)g

m = 50x11.3 / 10.3 = 54.8 gram

If the lead piece is fastened to the top of the block when the block just floats the lead piece is above the surface of the water and no buoyant force acts on it.

Let this time the mass be m'

(200+m)g = (200/0.8)g

m = 50 gram