I can't tell if the letters are 'SN', 'NS' or 'N & S'. In any event, here's more:
And as PBK points out by reference:
S N or N S maker's mark on petal bells.
Seth North, brother of James North. Seth took over full operation of the bell foundry in 1803 after James returned to New York. The foundry closed in 1840. Seth's early bells have a U-shaped cast shank. His later bells have a drilled shank.
Seth North and Henry Stanley
N & S maker's mark on some larger petal bells.
Some have a 5-pointed star in place of or in addition to the N & S.
All bells have a distinctive sunburst design in each petal.
This New Britain, Connecticut, foundry operated from about 1830 to 1850 when it was sold to Russel & Erwin.
Don.....
Source:
http://www.classicbells.com/Info/bellMakers.html