From My Unattributed Pile..

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These are thought to be coins minted either officially or pseudo-officially on the fringes of the empire out of necessity. Perhaps they were used to pay soldiers on the extreme frontiers of the Roman territories or maybe to bolster the economy of regions far from the normal means of monetary distribution. Whatever the reason, many of these coins exist.

According to Doug Smith's postscript to The Vocabulary of Classical Numismatics, many Roman coins can be classified as "coins of necessity":

Of uncertain status are thousands (millions?) of surviving bronze coins of the Severan era that copy silver denarii. Some examples still bear traces of a very thin silver wash... Termed Limes (Lim-ace) or coins of the borders, these may be another example of coins of necessity. They may also be officially sanctioned issues for use in regions where political unrest made it hazardous to ship large amounts of silver. These low value issues could have served troops on the front and been redeemable for good coinage when they returned to the stable regions.

The name "Limes Denarius", although a misnomer, has been applied to these so often that it has stuck. Here again, although we know that these AE denarii are more or less faithful copies of silver prototypes, and we know that they were both struck and cast in various places - we even have numerous molds and forgers' dies - we don't understand the role, if any, they may have played in the official monetary system. Were they copies made by semi-Romanized folks just outside the reach of empire? - folks who had become accustomed to the use of coin but who did not have access to official supplies? Were they a form of military scrip meant to keep large quantities of precious metal from falling into enemy hands in the event of a defeat - and presumably redeemable in good coin at some future date? Were they out-and-out counterfeits? Were they particularly debased official issues? (well, the cast ones probably weren't) Or did they fill some, as-yet unknown function? They might have done any or all of these at various times and places.

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This enigmatic little piece belongs to a group of coins referred to as limes denarii, limes falsa or AE denarii. There are numerous theories as to their purpose. Some believe they were minted in the outskirts of the Roman Empire to circulate there. Some contend they are non-Roman imitative (barbarous) types. One theory I have is they were made for the soldiers defending the outer regions of the empire, paid to them in lieu of silver, in the event the territory was lost. This would not have not enriched an enemy victor of a lost region with silver or gold. With my theory, these AE pieces could, if a soldier returned to a main part of the empire in retirement or if an area was forfeited, could in turn be redeemed for equivalent AR denarii, just as US soldiers used scrip during foreign wars. We will probably never know their purpose, but they can be found in excellent condition and are very collectable.

Another unusual aspect of this piece is the obverse legend. No such legend was used on official Roman coins, but it has been proposed this piece may have meant to read IMP ALEXAN_DER PIVS AVG, but the P is missing in PIVS.

Something to keep in mind is that some limes denarii do not have official Imperial equivalent types. Or, perhaps, they simply have not been found yet, but I have had examples that if they were silver, they would be unlisted in RIC.