It's really hard for me to tell, but it looks more like a D if I had to guess. It would also be a fair assumption given that I bought it out of a junk silver bin and I doubt a dealer would let a 1909-O slip into there.
I see the area that you are referring to but I do not think it is a mintmark at all because it does not appear to have any relief. I just checked a dozen mintmarked FR2-G4 Barber quarters in my silver stash and all of the Os and Ds were bold mintmarks.
"Buy the Book Before You Buy the Coin" - Aaron R. Feldman - "And read it" - Me 2013! ANA Life Member #3288 in good standing since 1981, ANS, Early American Coppers Member (EAC), Colonial Coin Collectors Club member (C4), Conder Token Collector Club member (CTCC), Civil War Token Society (CWTS) member, Liberty Seated Collectors Club (LSCC) & Numismatic Bibliomania Society member (NBS), USMex, Member in good standing, 2¢ variety collector.
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