Sidney the Skull and Crossbones

The depth and detail indicate that this represents just what we see, i.e.  a carved bas-relief memento mori (requiring a heavy duty frame and wall fixing if in stone!) and not the emblem on a pirate flag that I had taken it to be for many years! So, is this an actual spook then or just plain spooky? The skull is strangely lop-sided, as it is on all the examples I've seen. Stewart's was missing a part - note the skilful repair!

Thanks to The Phantom Highwayman for these wonderful pictures!