It's made of laminated steel plates riveted together and ground to a round shape. There should NOT be any ridges on it (unless you are looking at old tar residue).
Since you have determined the old coil is bad, cut through it with a hacksaw, down to the steel, then peel the remains of the coil of the core. If you get rough with it trying to press it out of the old coil, you will deform the riveted together laminated pieces and render the core useless. You do NOT want to know how I know this!
You may want to have a look at the official service manual at the link below: (page 46)