After extensive internet searching, I cannot find any confirmed information whatsoever on the Nathan Rubinstein who wrote this piece. There are two rather well-known people with that name: a prominent rabbi born in 1852, and a Romanian Jewish writer (? he wrote at least one book, it appears) who lived from 1913 to 2004. I cannot find any evidence linking either of them to any musical compositions, especially one titled in German (the 1913 guy emigrated to Israel in 1950, apparently). WorldCat does not appear to turn up any relevant results, and Google was inconclusive for both web and book searches (I did my best to narrow down both types of searches as much as possible). In fact, the only online source linking the name Nathan Rubinstein to this work at all is a
German online music store with a contents listing for a compilation of violin music assembled by Savva Shalman, for whom I cannot find dates either (though the publication seems to be from the 1980s?).