Group photograph of the SDP, Department Rotterdam, Congress, 1911, with Sam de Wolff, <a href="https://en.wikipedia.org/wiki/en:Willem_van_Ravesteyn" class="extiw" title="w:en:Willem van Ravesteyn">Willem van Ravesteyn</a>, <a href="https://en.wikipedia.org/wiki/en:Herman_Gorter" class="extiw" title="w:en:Herman Gorter">Herman Gorter</a>, <a href="https://en.wikipedia.org/wiki/en:David_Wijnkoop" class="extiw" title="w:en:David Wijnkoop">David Jozef Wijnkoop</a>, <a href="https://en.wikipedia.org/wiki/en:Louis_de_Visser" class="extiw" title="w:en:Louis de Visser">Louis de Visser</a>, <a href="https://en.wikipedia.org/wiki/en:Gerrit_Mannoury" class="extiw" title="w:en:Gerrit Mannoury">Gerrit Mannoury</a> and Jan Ceton
Joh. B. Egner ('s Gravenhage), (biographical data unknown, only mentioned once or twice in works from the 1910s)
1911
<a rel="nofollow" class="external text" href="https://resolver.kb.nl/resolve?urn=urn:gvn:IISG01:ADV10696322">Sam de Wolff</a> at geheugenvannederland.nl