30a is one of those mentioned in the preamble that is in Collins and not Chambers. It's a 7 letter "pattern" or 'prototype', 'model' etc around single letter "square" - definition is "part of crab's head".
Strangely stuck. Confident of my grid fill and found the C-to B name easily. Presume the A-to-B is the obvious name to go with that, but cannot find a path through the relevant letters.