The grammar of a model is a set of descriptions or specifications that tell us how to construct and interpret representations of the model. The syntax of a symbol or representation or model is its structure, its form, its shape. The semantics of a symbol or representation or model is its meaning, its content. We can, however, separate our discussion of these aspects. That's what happens in the following.
Notation is a part of a model's syntax.
A model has a basic set of symbols. It is 'rigid' with respect to that set of symbols. And that's it. PropL has those symbols and no others. If we stipulate further symbols, we either change the nature of PropL or we are creating a new model.