Theo van Gogh. The writing appears to say "<a href="https://en.wikipedia.org/wiki/nl:Cor_van_Gogh" class="extiw" title="w:nl:Cor van Gogh">Cor van Gogh</a>".
<div class="fn value">
De Lavieter &amp; Co.</div>
May 1878<div style="display: none;">date QS:P571,+1878-05-00T00:00:00Z/10</div>
<a rel="nofollow" class="external text" href="https://resolver.kb.nl/resolve?urn=urn:gvn:VGM01:b4793">Memory of the Netherlands, From Vincent van Gogh: letters, art, and context of the Van Gogh Museum</a>