Should I discuss this with my manager now or in the yearly review?
Bring it up now.
Anything that bothers you that much should be brought up with your manager reasonably quickly. Waiting until your yearly review would push the issue too far out - perhaps so far that the details have been forgotten. In addition, if it bothers you this much, there's no sense in letting it fester for months.
If you have a regular one-on-one meeting with your manager, that would be an appropriate time for this discussion.
If not, just say something on the order of "Hey boss. Do you have a few minutes to chat? I have something that's bothering me that I'd like to discuss."
During your conversation, try to stick to the issue that bothers you. In your case, it sounds like you are bothered by the fact that the position was initially targeted at one grade higher than you chose to accept.
Remember that there might be many reasons why this happened. It's possible that they re-graded the position before you were interviewed. It's possible that they thought they needed someone with more experience and abilities than you currently possess, but that they view you as capable of eventually getting to that level. Also remember that you accepted your current compensation, and presumably were content until you happened to see the old internal job posting.
Try to go in with the attitude that you just want to understand what happened, rather than just "I'm frustrated". After all, you did accept the offer for several reasons.
And try to be clear in your own head now what you hope to accomplish from your discussion with your manager. It's unlikely you'll be bumped up a grade just because you became frustrated - that's not likely to be a reasonable expectation.