Again, Notrab, I think your observation is relevant, though I don't think the colour plays any further part in the solution. The warning combines the replaced letters in one column which leads to the start of the work area.
Dis, I think you are right, the exclamation point remains (the I is discarded) which means that 49a is no longer a real word or name. I suppose the end of the preamble is referring to this, in an unwieldy kind of way...