Z = np.arange(9).reshape(3,3).astype(np.int16)
index = 1,1
print(Z[index].tobytes())
b'\x04\x00'
offset = 0
for i in range(Z.ndim):
... offset + = Z.strides[i]*index[i]
print(Z.tobytes()[offset_start:offset_end]
b'\x04\x00'

Should be

Z = np.arange(9).reshape(3,3).astype(np.int16)
index = 1,1
print(Z[index].tobytes())
b'\x04\x00'
offset_start= 0
for i in range(Z.ndim):
offset_start+ = Z.strides[i]*index[i]
offset_end = offset_start + Z.itemsize
print(Z.tobytes()[offset_start:offset_end]
b'\x04\x00'