The 1842 is indeed the date of the percussion conversion. The script S
should be followed by an a or German oe (=ö) and indicates the location of
the conversion. Sa means Saarn and Sö stands for Soemmerda (the factory
where Dreyse also built the famous needle gun did conversion work, too). The
other mentioned number is presumably the so called journal number of the
conversion factory.
The unit mark IR stands for Infanterieregiment (Infantry). The guns were
later second class and many bear later (additional) LWR marks for
Landwehrregiment (older soldiers of second classed units). The rifled
versions were still in use during the war of 1866 between Prussia and the
German Southern States and Austria.
Best,
Marc