I think Schleswig-Holstein is a very good guess. Over the centuries it has been schuttled back and forth between Danish and "German" ownership, the last change was in 1864. Danish is still an officially recognized minority language in Germany for this reason.
Stadardized spellings of anything weren't "standard" before the early 19th century, and this must be taken into account with names.
Someone else has suggested the Netherlands, and this also would be a sound suggestion. The spelling could come out of the Friesian area and language, but this is just conjecture on my part.