That looks like it will be a good usable stone for you once you lap it clean. Don't take off any more than you have to on that yellow side since the yellow layer is quite thin to start with.

The thing I like about lapping vintage stones is: pencil grid schmencil grid! The grime when it's gone will tell you you're lapped. Then, I do usually do a pencil grid as added measure and when that's off, I've got a flat stone.

Chris L