If you look at first two pictures in post #3 they basically have your answer in them. The first picture shows the lock mechanism typically of earlier machines. The second picture does not have that mechanism and isn't suppose to have it. You can tell by looking at the bracket for playfield support prop. If you notice in the second picture the side of the cabinet without the prop rod has a bracket just like the prop rod bracket. Those two brackets are what support the playfield and it is locked down with the lock bar once they made that change over.
If you look under the center of the playfield on the playfield without the lock mechanism you will see an angle bracket with a hole in it - that bracket is locked into place with the lock bar receiver once they got away from the lock mechanism used on earlier machines.