CIMRM 827 - Denarius recut with inscription. St. Albans, Britain

From: Verulamium museum:

This could be a protective amulet (lucky charm) or an admission pass to a temple of Mithras. .... It has been made by rubbing off some of the details of an earlier silver coin of the Emperor Augustus.

See Roger Beck, "Mithraism since Franz Cumont", ANRW 2, p.2049:

Finally one should mention the most interesting adaptation of a denarius of Augustus found at Verulamium (CIMRM 827) (Pl. XXII). The obverse has been entirely obliterated and replaced with the Greek legend MITHRAS OROMASDES in a circle with PHREN in the centre, while the scene of Tarpeia overwhelmed with shields on the reverse has been left (without its legend) as a representation of the rock-birth. Presumably it was the chance similarity of motifs that suggested the adaptation. The equation of Mithras with Ohrmazd, the supreme Iranian god, is of course highly significant, as is the use of what is most probably the solar epithet from Graeco-Roman magic Phre(n). Unfortunately, though, one cannot generalise to a universal Mithraic doctrine on the equation from what may be just the speculation of a single rather imaginative initiate.

For "Phrē", see H. D. Betz, The Greek magical papyri in translation, vol. 1., p.338:

"Phre: See Ra." "Ra (Re, Phrē): The Egyptian god of the sun (P)rē, without the definite article. Re means simply 'sun', while Prē, the form of the god's name from the New Kingdom onward, means 'the sun'. See Bonnet, RARG 626-30, s.v. 'Re'."

CIMRM entry

827.
Round silver denarius of Augustus (diam. 0.022; w. 2.38 gr.), found in St. Albans under the walls of the building IV, 1, dating from the second half of the second century A.D. Verulamium Museum.
Mattingly in Num. Chron. (S. V) XII, 1932, 54ff with fig.; Wheeler, Verulamium, 211ff No. 8 and Pl. LXVI; Corder, Ver. Mus., 17 and fig. 10. See fig. 22l.
"A silver denarius of Augustus, the reverse of which bore the figure of Tarpeia overwhelmed by the Sabine shield, had had its legend smoothed away. In place of the obverse a fresh inscription had been incised: MIΘRAC ΩROMASDΗC, around a circle, and ΦRHN across the centre, the representation of Tarpeia being left to depict the birth of Mithras from the rocks."

I.e. MITHRAS OROMASDES and PHREN.

See also Spocking Fives and a Mithraic token. Currency defacement ancient and modern, 4 Mar 2015, which adds:

Ancient parallels for the practice do exist, however. During excavations of a Roman building in St. Albans in Britain (ancient Verulamium), an object (shown left) was found under Building IV (the floor dated to the second century AD). It was a silver coin (denarius) that had been altered, similar to the Canadian note above. The coin had originally been a denarius of the emperor Augustus dating from 19-4 BC, showing the portrait of the emperor on one side, and part of an ancient Roman legend on the other: Tarpeia being crushed to death by shields. What the coin originally would have looked like is shown below.
The writing on the coin (naming the moneyer responsible) has been erased, as has the obverse. The portrait of Augustus was removed, and instead a Greek legend was inscribed on the coin (RIB 2408.2): ΜΙΘΡΑC ΩΡΟΜΑCDHC ΦΡΗΝ (Mithras Oromoasdes (Ormuzd) Phren). The edge of the coin is inscribed D M (D(eo) M(ithrae)): 'To the God Mithras'. The coin was thus converted into an object in honour of the god Mithras, a god which came to Rome from the East (Ormuzd was the chief Persian god, and Phren was likely a sun god). The reason this coin in particular was chosen for conversion was again because of its imagery: myth told that Mithras was born from a rock, and so the image of Tarpeia being crushed by shields could easily be reappropriated into an image showing the deity's birth. The date of the find (well after the coin was struck) suggests that the coin may have been quite old when it was converted, though it still will have been legal currency. The process of conversion, however, would have meant that this silver coin could no longer function as money in the Roman world. The object thus represents a sacrifice of wealth: Mattingly suggests it was perhaps a token to gain admission to Mithraic worship, or to show membership of a particular level of the Mithraic cult.
Bibliography: H. Mattingly (1932). "A Mithraic tessera from Verulam." Numismatic Chronicle 12: 54-7.