Marketing booking classes do not necessarily map to the operating carrier's booking classes. For example, a TATL UA P fare being codeshared on AC will usually book into Z. I say "usually" because I haven't found a way to determine ahead of time what the underlying booking code is on the operating carrier.

In your case, since all premium cabin fares on AC earn miles, you can feel safe booking anything and you'll earn miles on it. What cabin you'll end up in might be an adventure, though