Dating a Seamaster from serial number

Hello All,
I'm a newbie on here so please bear with me if my question seems banal. I'm trying to check the provenance of a particular watch, Ref: 2231.80.00 which is a Seamaster 300M auto in titanium. The warranty card says it was sold in Oct 2000, the S/No. which is in the correct place, reads 8020****
I want the watch to be a pre co-axial 1120 ideally, I think that movement changed to the 2500 calibre around 2006 on but isn't the serial number a bit too late or could it be one of the last 1120's?
I'm sure it's not a fake but without opening the case to check the calibre etc all I have is the serial number & the card which seem to contradict (to me).
Many Thanks

Not sure there is an issue here. My 300m has a s/n that starts with an 8, and is from 2000. Was bought by me in 2000 from Selfridges in London. Hoping mine is legit! Been serviced s few times with no issues and contains a cal 1120. Bullet proof.