Congratulations on your Autoharp(tm). It is unique among
semi-automatic, "commercial folk" instruments in that it has been
in continuous production since 1890 right up through the present day.
Autoharps are common in the US, where they are still used by some folk
and bluegrass singers.
Originally intended to be played flat on a table or on the lap (as you
suggested), autoharps are now usually held flat against one's chest,
plucked with the right fingers, and "fretted" (pressing the chord bars"
with the left. Modern players not only strum chords, but also pick out
melodies and counter melodies on the strings. Thumb and finger picks
are worn like rings.
The Oscar Schmidt Corp. still makes them, usually with 15 chords.
In the US several small folk-style builders produce custom models with
different tunings and chord sets.
The odd notations on your strings are by Mr. Zimmermann, the inventor.
He used numbers 1-7 for the notes C-b, with extra markings for sharp,
flat, and for various types of chords. Modern 'harps usually omit
these, but I have a late 1890s model with the odd chord notations.
In the US you can buy a whole new set of strings for about $50. I have
replaced a few bass strings with guitar strings, as you suggest. But,
before trying to tune the instrument up to pitch, make sure the glue
joints are sound. Most old autoharps have the lower hitch rail block
pulling away from the bottom of the case, and need some serious gluing
and screwing before being playable. Enjoy it.