Granger:
Here is a Purdey with Island Locks. Note the lock is separated from the action by wood.


Here is a full bar action lock with the main spring pin clearly visible in the bar of the lock plate.



Here is a back action bar-lock where there is no visible main spring pin in the bar of the lock plate.
The gun was a Pigeon style hammergun and back action locks were used for maximum strength in the action bar.

The shape of the lock plate is irrelevant, look for the main spring pin.