Got a full grid fairly quickly then spent ages trying to figure out the final operation. I have a method that seems to follow the instructions but the problem is that quite a lot of the grid is effectively discarded--this to me seems to contradict the premise of the preamble that you could treat the "original" grid in such a way that you end up with the "modified" grid (i.e., the one we as solvers started with), because where would those extra letters come from? I'm not sure I'm thinking about this the right way, any nudges welcome!