Sounds like you found a Catholic sacramental and not a relic at all!
The three holes in the cross were there to hold the corpus attached the cross. The corpus being the body of Jesus on the Cross.
OF THE LETTERS, WHICH ARE INSCRIBED ON THE MEDAL
Besides the two Images of the Cross and—St. Benedict, there are also inscribed on the Medal a certain number of Letters, each of which is the initial of a Latin word. These words compose one or two sentences, which explain the Medal and its object. They express the relation existing between the Holy Patriarch of the Monks of the West and the sacred sign of the Salvation of mankind, at the same time that they offer the faithful a formula, which they may make use of, for employing the virtue of the Holy Cross against the evil spirits.
Those mysterious letters are arranged on that side of the medal on which is the Cross. Let us begin by noticing the four which are placed near the Cross, one at each of the outward corners: C S P B that is: Crux Sancti Patris Benedicti; in English: “The Cross of Holy Father Benedict.” These words explain the nature of the Medal.
On the perpendicular line of the Cross itself, are these letters: C S S M L they stand for these words : CRUX SANCTA SIT MIHI LUX; in English: “May the Holy Cross be my Light.”
On the horizontal line of the Cross are these letters: N. D. St. M. D. the words which they imply are: NON DRACO SIT MIHI DUX; in English: “Let not the Dragon be my guide.”
These two lines put together forum a pentametre verse, containing the Christian's protestation that he confides in the Holy Cross, and refuses to bear the yoke which the devil would limit upon him.
On the rim of the Medal there are inscribed several other Letters; and first the well-known Monogram of the holy name of Jesus, IH St. Faith and our own experience convince us of the all-powerfulness of this divine Name. Then below, beginning at the right hand, the following letters: V. R. St. N. St. M. V. St. M. Q. L. I. V. B. These initials stand for the two following verses. VADE RETRO, SATANA; NUNQUAM SUADE MIHI VANA. SUNT MALA QUAE LIBAS; IPSE VENENA BIBAS. in English: “Begone, Satan! And suggest not to me thy vain things; the Cup thou profferest me is evil; drink thou thy poison.”
OF THE LETTERS, WHICH ARE INSCRIBED ON THE MEDAL
Besides the two Images of the Cross and—St. Benedict, there are also inscribed on the Medal a certain number of Letters, each of which is the initial of a Latin word. These words compose one or two sentences, which explain the Medal and its object. They express the relation existing between the Holy Patriarch of the Monks of the West and the sacred sign of the Salvation of mankind, at the same time that they offer the faithful a formula, which they may make use of, for employing the virtue of the Holy Cross against the evil spirits.
Those mysterious letters are arranged on that side of the medal on which is the Cross. Let us begin by noticing the four which are placed near the Cross, one at each of the outward corners: C S P B that is: Crux Sancti Patris Benedicti; in English: “The Cross of Holy Father Benedict.” These words explain the nature of the Medal.
On the perpendicular line of the Cross itself, are these letters: C S S M L they stand for these words : CRUX SANCTA SIT MIHI LUX; in English: “May the Holy Cross be my Light.”
On the horizontal line of the Cross are these letters: N. D. St. M. D. the words which they imply are: NON DRACO SIT MIHI DUX; in English: “Let not the Dragon be my guide.”
These two lines put together forum a pentametre verse, containing the Christian's protestation that he confides in the Holy Cross, and refuses to bear the yoke which the devil would limit upon him.
On the rim of the Medal there are inscribed several other Letters; and first the well-known Monogram of the holy name of Jesus, IH St. Faith and our own experience convince us of the all-powerfulness of this divine Name. Then below, beginning at the right hand, the following letters: V. R. St. N. St. M. V. St. M. Q. L. I. V. B. These initials stand for the two following verses. VADE RETRO, SATANA; NUNQUAM SUADE MIHI VANA. SUNT MALA QUAE LIBAS; IPSE VENENA BIBAS. in English: “Begone, Satan! And suggest not to me thy vain things; the Cup thou profferest me is evil; drink thou thy poison.”