The Gallic usurper Postumus ceased striking sestertii and double sestertii in the year 263. Unofficial mints continued the production of double sestertii.
Unofficial mints continued the production of double sestertii seemingly until towards the end of Postumus’ rule in 269. The imitations declined in size so much that the radiate bronzes are frequently mis-described as dupondii.
Initially the imitative coins continued the designs used for the official bronze coinage but, through time, additional reverse types entered the repertoire. The majority of the new reverses sought to mimic the reverse types of the official radiate silver coinage of Postumus, for example PAX AVG is frequently encountered.
Occasionally other coins serve as prototypes. During the 250’s Gallienus struck a silver antoninianus at the Trier mint with the reverse of two captives sat beneath a trophy of arms accompanied by the legend GERMANICVS MAX V. The reverse design, including the legend, can be found on this pair of significantly reduced imitative double sestertii of Postumus, perhaps manufactured c.267-8.
These two coins quite clearly share the same reverse die, the obverse though, are from different dies. I know of another coin that uses this reverse die and is also an obverse die duplicate with the bottom coin above. It was found in a hoard of Postumus coins from Mericourt-l’Abbe.
The Mericourt-l'Abbe hoard comprised official, unofficial from “atelier II”, now thought to be a significant unofficial mint complex making (striking and casting) both silver and bronze coins of Postumus located at Chateaubleu, and other irregular coins of uncertain mint location.
You might speculate that what the die cutters were aiming for was a similar reverse type that did occur in the official large bronze of Postumus, the captives and trophy but with the legend FELICITAS AVG. The fact that they legend used was from a coin of Gallienus may demonstrate that the overall design was more important than accuracy of the accompanying lettering.
