The signature of Antonis Benakis, Greek art collector and  founder of the Benaki Museum in Athens, Greece. From a certificate issued by the Royal Yacht Club of Greece, which Benakis signed as the Club President.
Antonis Benakis (1873-1954) // Αντώνης Μπενάκης (1873-1954)
From a old certificate on sale on www.ebay.com