Nice collection of the final sestertii to be issued @Cucumbor ! Gallienus was the last emperor to do so, though Postumus may have issued a few double sestertii in the months following Gallienus' death and his own demise in 269.
I don't know the details but I suspect that the Gallienus sestertii were largely issued during the earliest years. Postumus issued a lot of bronzes including many smaller ones sold as dupondii which I wonder if might have been later (double) sestertii and barbarous copies struck who knows when. There are a few bronzes during the time of Aurelian which are sometimes called asses due to their size and sometimes called sestertii. I don't know and do not know if the experts have hard evidence or just educated guesses. I encourage all who care about this period to investigate as much as possible. My recollection of things I don't collect is faulty at best. Postumus Aurelian Severina