laminated, old school brass, non magnetic, does not have the weird core clip like the st ives, that would be a pain to have made,
don't want to clamp the core too tight, as it expands and contracts, of you limit this movement, distortion takes place,
"Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.
This effect causes energy loss due to frictional heating in susceptible ferromagnetic cores. The effect is also responsible for the low-pitched humming sound that can be heard coming from transformers."