Them. Pappavangelou was a Greek collector active in Manchester, UK in the early 20th century. He is attested by a Turkish coat acquired by the Metropolitan Museum of New York in 1912 (12.127: https://www.metmuseum.org/art/collection/search/446220). He is mentioned as owning 40 gold coins of Lysimachus in the archival documentation for IGCH 148.
M. Ritsos was a collector active in Athens in the early 20th century. He was the attested owner of the Ganymede jewelry, now in the Metropolitan Museum of Art (37.11.8-17: https://www.metmuseum.org/art/collection/search/256975). His numismatic collection is cited in coin hoard research notes.
Euripides S. Sepheriades was a coin collector active in Athens in the 1950s and 60s. He is mentioned in the preface to "Euboian League and its Coinage" by William P. Wallace (1956) and in the provenance for two coins (62.568: https://collections.mfa.org/objects/3519 and 63.1518: https://collections.mfa.org/objects/3554) acquired by MFA Boston in 1962-3.