Mary Bowden was the first person to be buried in the Old Cemetery on 4th February 1856. You will see that she was buried in consecrated ground in plot F51 in an unpurchased, “unbricked” grave.
An unbricked grave is where the soil is put back in after the interment on top of the coffin whereas a bricked grave is literally bricked (like a vault) sometimes painted then the soil is put back into the grave on top of the bricks. A bricked grave was a sign of wealth. If a grave has not been purchased then no headstone can be erected as there is no owner to authorise the work. (Note: A paupers or public grave is a grave that has not been purchased.)
The plot numbering can be confusing if you do not know whether the ground is consecrated or not because the section letters are reused.