The frame
The chronology list of the Met's timeline for Mesopotamia names "Hellenistic rule: Macedonian and Seleucid dynasties, ca. 331-138 B.C." The entry for 331-138 B.C. opens with the defeat of the Persians by the armies of Alexander of Macedon; after Alexander's death in Babylon in 323 B.C. his successors divided the empire, and Syria, Mesopotamia and Iran fell under the rule of Seleucus. According to the same entry, the empire was governed from two capitals, Antioch in Syria and Seleucia in southern Mesopotamia, and the newly established cities were laid out after the Greek style and largely populated by Macedonian and Greek colonists. Source 1.
Boundaries and overlaps
The end of the period is the Parthian conquest, and the sources differ on its year. According to the timeline, the Parthians from Iran under Mithridates I (r. ca. 171-138 B.C.) seized Mesopotamia from Seleucid control, and the chronology list starts Parthian rule "ca. 138 B.C."; but the Parthian key-events entry on the same page opens "ca. 139 B.C.". The Kelsey Museum's page on Seleucia states that "in 141 BCE, the Parthians under Mithridates conquered the city". The site ends the period in 138, following the chronology list, and keeps 141 and 139 visible. The start has no single date either: the page does not date the transition from Macedonian to Seleucid rule. On the site's Iran timeline, a separate Seleucid page ends much earlier, in 247 BCE, where the Parthian period begins there. Sources 1, 2.
On this site
One item is placed in this period: the article "Seleucia on the Tigris: a Hellenistic city in four levels", based on the Kelsey Museum's page and its Level IV, 307-141 BCE, "in which the city was a Seleucid capital". The article is also placed in the Parthian period, because the same source dates levels from the time of Parthian rule. Source 2.
Gap: no second item was found. The Met's timeline mentions "other newly established cities" without naming them, and the only Mesopotamian one it names is Seleucia; UNESCO's page on Babylon says the property includes remains "until the Hellenistic period" but neither describes nor dates a Hellenistic phase; the Kelsey page deals with Seleucia alone. Missing, precisely: a second site in Mesopotamia with a Macedonian or Seleucid phase dated by excavation, such as an excavation report of a Seleucid level at another site. Sources 1, 3, 2.