We could perhaps say that by "2" B means "the way B manipulates the symbol '2'", but that doesn't seem to buy us anything.

I think the meaning of a concept-detector (the concept it detects) in the more naive sense should be considered in the context of an agent, and refers to the way this concept-detector should be, as far as the agent can see (i.e. it's a question of knowable self-improvement). And the way it should be of course depends on how its properties control agent's actions, i.e. on the role the symbol plays in agent's algorithm. (This promises simplification in so far as the context-detector is blind to its purpose, i.e. its usability and direction of refinement is seen only in terms of other instrumental heuristics.)