The souvenir purports to be from the oak at Boscobel House (then the home of the Catholic Giffard family), in which Charles II hid after his defeat at Worcester (1651.)
It's a keepsake, to preserve a memory - but whether to commemorate a noble Kings' survival with the help of God and the true English (oak = England, etc.) or coward's skulking away in the dark (dank hollow, then priest's bolt hole) is not clear.
The inscription's being in abbreviated Latin suggests it's possibly more likely Royalist, and its conjectured arrival back in England by way of France would support that idea.
Either way, it's an example of the exercise of propaganda; an attempt to influence with words through an object partway between religious relic and tourist trophy.