I'm fettling a No. 5 1/4 plane and had a good look at the iron. It has a potato chip quality to it with a wave from front to back and a slight cup at the edge -- bending down at the sides. I milled the chip breaker straight across and the chips just jam into the corners
I really like junior jacks and wouldn't have any problem getting a modern thicker replacement iron. Still, I'm up for some percussive maintenance with nothing to lose. Usually I'd shim one edge on the anvil and give it a couple light smacks with a heavy BP hammer. If I can get it closer to flat, the stones will finish the job.
If anyone has a better way to reason with that cup, I'd love to hear it. I'm not so worried about the front-to-back wave since the iron always deflects under the tension of the breaker screw anyway. It's that first inch I need to get right.
Thanks and cheers, sh