No, I 've never found their serial numbers to work like that. If you join the owners group(see the posts earlier this week) you can look up the build date from the serial number. Otherwise you can look at the date stamp on the wheels to get a rough idea of age.
I thought that 2005 was the first year with the blue canvas, perhaps it's a 2004 or earlier model that was sold in 2005 so the owner believes it's a 2005. I don't know much about the 575 models so I don't know what other changes were made around that time.
Rob
PS Black Country Caravans have a 2005 575 for sale ....and it is blue.
Model years wont always begin and end on Jan 1st. Most will be September when the shows are about. If that's the case you could have an early 2005 and a late 2005 with different canvas.