It appears to be MMS-009 or "Blob S". Here is a great link to the Lincoln series "S" mint marks. http://www.lincolncentresource.com/San_Fransisco_Mintmark_Styles.html
About that site. Is it implied that for the years not shown the previous marks carried until the next change is noted? For example, would ant 1953 thru 73 S cent have only one of the 2 marks shown for 1952?
Check out this page on CONECA. It tells every year each type of S was used and if it is a scarce variety or not. http://varietyvista.com/San Francisco Mintmark Styles.htm