Intriguing gravestone

Old graveyards are fascinating places. Spent half an hour wandering round the cemetery beside Stirling Castle today.
A lot of stones caught my eye but I was intrigued by what these symbols meant. The best guess is that they were tools used by a stonemason or some such tradesman.
The gravestone dates to the 1700s. If anyone has any idea about the symbols I'd be glad to hear it.