Opinions needed on this one. Could this possibly be an "S" over a "D" or just damage? You can see where it was hit but there is just a small piece on the left bottom of the top curve of the "S" that could be a "D".
I don't think so. The top looks like it took a hit. Prior to 1989 (or maybe it was 1990) all mms were hand punched into the die. The MM may have been punched a little deeper and the recessed area inside the loops of the S on the punch show as the raised areas inside the loops.
74 had 2 different MM styles that were used, yours is an MMS-009. It looks as if the MM took on some PMD. http://varietyvista.com/01b LC Doubled Dies Vol 2/San Francisco Mintmark Styles.htm
I didn't know there was such a variety. Are you claiming this is a discovery coin? This page will answer a lot of questions: http://www.lincolncentresource.com/OMMS/OMMS.html