It could be an authentic first century box with an authentic first century inscription, with an authentic 4th century addition to make it more valuable as a religious relic.
After Helen, mother of Constantine announced that she would tour Palestine to research Christianity, there was a boom in creating relics for her to discover.
Perhaps you have heard of the Holy Lance, discovered by Peter the Hermit during the first crusade? Hitler touched it and felt that possession of it was key to his destiny. Recent tests reveal (Iron has carbon in it) it to be a 7th century fraud. As such, it is a relic of something, perhaps the first crusade, but not of Jesus. There is a nail in it that is consistent with a first century Roman nail, but of course there were quite a lot of them out there.
What really interests me more is this — Is James really the half brother of Jesus? Did Mary have other children after giving birth to Jesus, or did she remain a virgin for the rest of her life?