Thanks all!
They are of course special strikes with a great style, an unlisted bust type and no mintmark.
Actually, we can distinguish three different emissions with this reverse type struck by Maxentius for Constantine as Caesar at Rome.
1) AE fraction or "half follis" (about 2,5 gr), two specimen known. The reverse legend is PRINCIPI IVVENTV
TIShttp://www.notinric.lechstepniewski.info/6rom-165.html2) AE medallion or "reduced follis" (about 5,2 gr), one specimen known. The reverse legend is PRINCIPI IVVENTV
TI3) AE medallion or "reduced follis" (about 6,7 gr),one specimen known. The reverse legend is the shorter PRINCIPI IVVENTV
TI suppose the AE fraction comes first, whit its full, long reverse legend. It might be an earlier issue. ROME 125 has the same size and reverse type, but it has been minted for Severus II.
2 and 3 have some stylistic and metrological analogies with the CONSERVATORES VRB SVAE type of reduced follis (6-7 gr) and could be a later emissions.