On the 28th of September 351, the armies of Constantius II won the bloody battle of Mursa Major and Magnentius was pushed into Gaul with his forces all but destroyed. As a result, Constantius returned to rule on the mid Danube and retook Siscia. In anticipation of his tricennalia, he started minting gold and silver in his name and for Gallus Caesar very soon after retaking the mint. This specimen is part of that early coinage for Constantius II at Siscia following the victory of September 28, 351: AR21x20mm 3.07g silver siliqua, minted at Siscia, cca. late 351. DN CONSTAN - TIVS PF AVG; pearl-diademed head right VOT / XXX / MV·LT / XXXX inside wreath. SIS RIC VIII Siscia 325, R Rather medallic in look and weight, this siliqua issue is of particularly good style and workmanship. Although the volume of mintage must have been very large, as Constantius was preparing to invade Gaul and put a stop to the regime of Magnentius, this particular issue is rather scarce, probably recalled and reissued as reduced siliquae in the later part of the decade. This specimen is a particularly pleasing example of a scarce issue, minted for Constantius II very likely while the emperor was stationed at Siscia.
The silver was likely struck to pay the troops. Nice example of the type. If the Romans had not engaged in destructive civil wars during the 4th century then likely the West could have survived a bit longer. But with Honorius on the throne around 395 that might be an academic question.
Nice 'n' hefty! Cataloguers often don't distinguish the heavy ones from the later light ones. In fact I think you may have benefited from this in acquiring your specimen! I have a later, light (2.07g) version from Sirmium. Not rare and not as historically interesting, but at least it's pretty.
The weight was actually stated, what I did not expect was the wide and medallic flan. Not even the siliquae from the 330s and 340s were this large. Yours is also nice and you might sell it short by saying it's not historically interesting: the continuation of the tricennalia issue on the reduced siliquae very likely coincides with Constantius II being at Sirmium starting with 357. Considering that during the later years to 360 the legend changes to a quintricennalia dedication (XXXV), yours is about 357.