Island locks are surroiunded totally by the stock wood, ie they have no metal to metal connection with the action body.
Peninsula locks are attached to the action body. Both island and peninsua are back action locks, the main spring is in the back of the lock.
There are also the conventionally shaped back and bar action locks fyully attached to the action body.
Bar action locks have the spring forward, housed in a recess in the action bar. There are people who assert that they can tell a difference in trigger feel between bar and back action locks.
The two things I would insist for safety on a hammer gun more than spring location is that is should open with hammers cocked and it should have rebounding locks. Rebounding locks do not let a gun fire unless the trigger is pressed. In the event of an acciental hammer fall the tumbler is trapped by the safety notch built into the rebounding system.
