The Eikosachalkon denomination
The Eikosachalkon was a Ptolemaic bronze denomination worth 20 chalkous. It is expected to weigh ~28.5 grammes.

It is a fairly new denomination discovered through metrological analysis, and coins of this denomination are sometimes referred to as lightweight tetrobols.
An AE Eikosachalkon struck 246-235 BC in Alexandria
Obverse: Laureate head of Zeus-Ammon right

Reverse: Eagle with spread wings standing left on thunderbolt; cornucopia on shoulder; ΠTOΛEMAIOY_ΒAΣIΛEΩΣ, Λ

Diameter: 35.5 mm
Die Orientation: -
Weight: 31.1 g
Eikosachalkon = 5/2 Obol
Svoronos 1167, SEAR 8025