Galerie Nefer in Zurich is known to have handled material that has been returned to Italy by a major North American museum. For example the Douris phiale returned by the J. Paul Getty Museum (inv. 81.AE.213) was formed from a series of fragments acquired as purchases, gifts and loans in 1981, 1985, 1988 and 1992.
I happened to be checking details about the proprietor, Frieda Tchacos, and noticed that New York's Metropolitan Museum of Art had acquired a fragmentary red-figured pelike attributed to Euthymides in 1990: "Gift of Mrs. Frieda Tchacos, 1990" [MMA]. The accession number is given as 1990.305.
The fragment is noted in the Annual Report of the Trustees of the Metropolitan Museum of Art 121 (Jul. 1, 1990 - Jun. 30, 1991) 32 [JSTOR]. This accession number, 1990.305, is listed as a gift of Dietrich von Bothmer.
Tchachos gave a fragment of a red-figured cup attributed to Makron in 1990: acc. no. 1990.170. It appears elsewhere as "Gift of Mrs. Frieda Tchacos, in honor of Dietrich von Bothmer, 1990" [MMA]. The fragment joined a cup acquired by the MMA in 1979, acc. no. 1979.11.8, "Mr. and Mrs. Martin Fried Gift, 1979" [MMA]. Bothmer was able to add two further fragments in 1989 [MMA], and another fragment in 1994 [MMA]. When was this cup broken up? How and where did Bothmer acquire these pieces?