Waking up an old thread...

I want to fly LAX -> PRG; OSL -> BOS in the summer. Since I collect CO miles, I am interested in routing the outbound through one of LHR (on VS), AMS (on KL), or CDG (on AF) and then connecting to PRG on OK. One of the cheapest fares for this slightly odd routing, found during searches on ITA, is a YY fare, fare basis YHXAN1ME for LAX-PRG, which allows the LHR connection. These fares are mysterious to me, so I'm hoping someone knows the answer to the following question.

Question: ITA books this fare on VS and OK in Q class. Why does it use Q class and not Y class? Is this specified in some IATA table, or does it just use the lowest fare class available (so if I want a higher fare class, I should just wait?)? How do I find out what booking classes IATA fares like this one book into on different carriers?

Anyone know? I'd love it if these fares could book in Y, since the elite qualifying bonus miles for full Y would be hefty. But I'd settle for just understanding what booking class these fares do use.