Many thanks for your reply.
So this specimen could have been stroke just before the opening of the Siscia's atelier?
There are specific works about the early chronology of Siscia mint under Gallienus?
I have both Normanby Hoard book and M. R. Weder "Mint forgeries of Siscia and some dates of Gallienus" that have some considerations about this mint, but may it be more helpful to find some specific works.
For example this coin (from Siscia) has a similar style: