I was thinking more about inconsistency and non-uniformity in the material, thickness, magnetic islands etc. I'm guessing that the cartridge would pick that up. I really don't know
There was a documentary on one of the Discovery channels about OWA and the recycled iron they used to manufacture the turntables. There was lots of good science and some bad pseudo-science but carefully selected old iron, properly resmelted under controlled conditions and cast into high precision moulds should eliminate many of those issues. I have forgotten my metalurgic chemistry so not sure what happens to the magnetic fields but I seem to remember they orient N/S on cooling and retain that memory - much like much of the subsurface geology on the planet on which we sit.