Trying to book myself some cheap biz class flights using aa.com.

Last time I tried this it was an IB codeshare that was throwing up an I-booking fare and then giving my S booking class.

This time it's mainline BA (LHR-HAM) - aa.com is showing a fare basis of DEUNBA on both legs, but when you progress with the booking it shows a booking code of Y and S.
[for completeness: this is choosing 'economy' on aa.com; choosing biz restricted brings up CEUNBAx2. And from experience, aa.com will put you in biz when you select economy if it's cheaper than economy. V useful when there is I availability up front but only Y down the back, and you're on a Y only policy ]

Last time with it being an IB codeshare I could see a fare basis of I being economy, but how is a fare of DEUNBA on BA booking into Y ?

Can anyone shed any light in the next half hour before I have to go ahead and book ?

ta