Can be a number of things. Raw (un-vulcanized) or "gum" rubber, vulcanized rubber and other elastomers with or without abrasive fillers, plasticized pvc or vinyl. The Staedtler-Mars erasers are most likely a filled vinyl, from what's available on the web.
My own experience has been confined to traditional vulcanised natural rubber based erasers intended for pencils and inks, and in these a principal ingredient has always been the abrasive pumice, with higher levels being added in an ink rubber. For 'soft' graphite the usual contenders include the kneaded or 'putty' erasers. The composition of these can vary quite markedly with polyisobutylene or plasticized PVC being found in some instances, and uncured rubber in others. One ingredient that has been used in rubber systems is factice(vulcanized vegetable oil) which has friable characteristics. Tackifiers may also been present to assist the adhesion of the graphite particle away from the substrate but there can be a risk of smearing.
