The old altar pillar with the inscription PENITENCE! PENITENSE! on top and MISSION 1891 at its base.

The 8 has been written with two circles that do not touch. Perhaps it is to point to a bible passage. M for mission could be for MARK, so the passage might be MARK 1:91 but there is no verse 91. However, Sauniere had the pillar placed upside down, so if we right it, it becomes 16:1 and MARK 16:1 reads: And when the sabbath was past, Mary Magdalene, and Mary the mother of James, and Salome, had bought sweet spices, that they might come and anoint him.On its own it means little, but if we take verses 1 to 8 or 9 - to keep in with the numbers we have here, it makes more sense and tells of Mary Magdalene arriving at the tomb to find Jesus has gone!

Incidentally, if the M of Mission points to Matthew 16:1 then it tells of a sign from heaven, which fits in with my theory that the statues, etc, that Sauniere installed, contain clues - or signs - leading to something.