INVERTED “S” MINTMARK? 1928 quarter?

Is this an inverted S mintmark?

Also what is the “M” to the right of the date?

 

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looks like a normal small S,https://www.ngccoin.com/variety-plus/united-states/quarters/standing-liberty-quarters-1916-1930/

M is designer initial,
Obverse Designer: Hermon A. MacNeil
Reverse Designer: Hermon A. MacNeil

 

There were two different mint mark styles used on the 1928 S SLQ. Yours does look like the small S but there is a listing for an inverted mint mark FS 501.
http://varietyvista.com/08 Standing Liberty Quarters/San Francisco Mintmark Styles.htm
Here's an old Coin Talk thread about it: https://www.cointalk.com/threads/1928-s-standing-liberty-quarter-inverted-mm.237745/

 

I spent several minutes comparing it to the one in Cherrypickers. This looks exactly like the example they provided.