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.
Eikosachalkon

(1) Ptolemy III 246-235 BC
Obverse: Laureate head of Zeus-Ammon right
Reverse: Eagle with spread wings standing left on thunderbolt; cornucopia on shoulder; ΠTOΛEMAIOY_ΒAΣIΛEΩΣ, Λ
Ref: Svoronos 1167, SEAR 8025