The 30mm Iron z32 calipers are identical to the ones that came on the s14 silvias. IIRC s15 brakes were the same as well. R32 and 33 calipers are slightly larger than the 30mm iron and aluminum z calipers. They resemble brembo calipers in design. Lastly you can tell the aluminum z calipers from the others because they have heat sink grooves in them. The iron ones do not, not to mention aluminum is not magnetic.