German linguist <a href="https://en.wikipedia.org/wiki/August_Leskien" class="extiw" title="w:August Leskien">August Leskien</a>.
The caption says «W. Str. fec.»; most likely the German linguist Wilhelm Streitberg (1864–1925).
1913 or earlier
<i>Indogermanisches Jahrbuch</i> I (1913) <a rel="nofollow" class="external autonumber" href="https://archive.org/stream/indogermanisches01indouoft#page/n7/mode/2up">[1]</a>