Is it causing problems?
I got a 2001 model ,600 and bought a battery last year, I think the numbers where yb something, anyway I bought a mottobatt battery far better find them online they will tell you which one you need, 50 quid I paid.
These are well known and a trusted brand.http://www.ebay.co.uk/itm/Yuasa-YT12B-BS-Battery-Yamaha-600-FZS600-S-Fzer-98-03-/271587682975?pt=UK_Motorcycle_Parts&hash=item3f3be32a9f