In the past (and now too I think), the official "year" of agriculural machines was established by the date of first sale of that machine to a retail customer. That made sense because manufacturers would put a model into production, make and warehouse a large number of them, and then gradually ship them to dealers to fill orders. Since machinery's value is more based on its ability to do work, a machine was considered "new" until someone started using it regularly.
The Agricultural market has managed to steer somewhat clear of the "title" too, making the year less of an issue. What really counts is when a machine goes into service. Manufacturers can't guess that at assembly time so they don't stamp it on tractors.
Many of today's tractor buyers are more callibrated to automotive marketing so the question comes up whether a tractor is a "new 2005" or a "left-over 2004" - but there may be no difference except how long the crate was stacked in a warehouse.
Take care, Dick B
"Suppose you were an idiot - and suppose you were a member of Congress - but I repeat myself." - Mark Twain