Time to disagree with the instructions again. The C15 has unusual curved top tanks. I assume that these were either to allows water to run off when they were overfilled, or just annoy model makers who find square tops a lot easier to build.
The sides are supplied as single etchings. On the back of the curve there are a series of half etched lines to make bending easier. So far, so normal.
In the book of words, the suggestion is that these bends should be left until construction is complete. Quite why you'd want to do this is a mystery since once the boiler is in place you won't be able to get at the metal to curve it properly. The only guide is the front face, there isn't anything at the back. I'd prefer to have a cage to work around but it's not essential otherwise I'd make one up out of spare etch.
The front face also shows the limitation of slot and tab assembly - the slot is slightly too far back and if I rely on it the tanks will have an unusual and wrong recessed front.