I believe it is filled MM, caused In some cases by small die break in others a deteriorated die, or a well used and deteriorated mint Mark punch. The mints hand punched MM's into working dies until mid 1980. There are several well known S mint marks types from 1979 and 1981 type I & type II which do carry a premium if type II This is a common occurance and worth very little if anything as an error AND in some cases not an error at all. By PSD were you referring to post strike damage? If so I would say No
