I heard a few D over S or D over D around this time and ofc the 1949 Mark over the horizontal. Not sure if this could've been caused by anything in circ, but it just seems like a strange one to me. Honestly i would love a D over horizontal S or even D and it's clouding my thinking... Opinions on this one appreciated as always!!
My honest opinion.. Looking at the condition of the Nickel I woulf say the Mint Mark just has circulation wear and tear.
As stated, most likely wear. There are no listed 1956 D or S OMM, on http://varietyvista.com/05 JN RPMs/Over Mintmarks.htm or on http://doubleddie.com/939955.html. There are some RPMs, but it's hard to tell from your pictures if this is one.
