Indeed, though I am certainly not expert on anything about BA, my online searches only yielded details about BA A350-1000 airframes. Nothing on the A350-900. But...

I saw a listing on American Airlines' website for AA flight #6948, which appears to be a codeshare operated by BA. LHR-AUS. AA shows the aircraft type as "359 - Airbus A350". Searching on the BA website, I find that corresponds to BA flight 0191, which they simply list as "Airbus A350 jet". I assume that the "359" designation (on AA.com) indicates it's an A350-900. But to be honest, I don't know if that's different from how they list the A350-1000. How are the A350-1000s listed? (just 350? 351? 35K?). I've also searched for the flight numbers but the only results that came up were for older (not currently operating) flights with non-A350 aircraft.

So, I'm speculating...but it says "A359". I suppose that could just be AA making a mistake (wouldn't be the first)...

In any case, it's probably not critical to me if it's an "older" A350-1000 (all of 3 years old) or a brand new A350-900 (with that "fresh from Toulouse smell"), I'm guessing that any seat in business class on a BA A350 is a pretty good one.

Hope that helps clarify. Thanks.

Edited to add: Well, I checked on ExpertFlyer, and they say it's an A350-1000, so AA probably just has it listed incorrectly. Figures.