Watch List notes:

The Roman Empire
Gaius, 37 – 41

Sestertius 37-38, Æ 25.73 g. C CAESAR AVG GERMANICVS PON M TR POT Bare head l. Rev. ADLOCVT Gaius, bare headed and togate, standing l. on platform, r. hand extended to five soldiers with shields and parazonia, four of them carrying an aquila; in exergue, COH. C 1. BMC 33. RIC 32. CBN 45.
Very rare. A lovely untouched enamel-like green patina with some very minor area of
porosity on reverse. A very detailed reverse composition, minor area of weakness
on obverse, otherwise about extremely fine / extremely fine

The reverse of this coin is shown in the standard reference book for Roman military equipment, 'Army of the Caesars' by Prof Thomas Fischer, 2012, on page 44 as these sestertii of Caligula provide the first images of the standard rectangular Roman scutum (shield).
In my opinion this sestertius may have been minted in an auxiliary mint in the West. It is interesting not only for the portrait style but also for the absence of the letters SC (Senatus Consultus), a clear political message of imperial power over the Senate. AMP.