For a transit bus called a 1990 series, does it more like mean that it entered service in 1990 or 1989?

I know the 2016 Honda Pilot and the 2016 Acura RDX are out right now, but I am starting to wonder after receiving this other answer for one of my other questions:

"They have rarely ever had model years. Just a "series" that may span many years. A few makers may have tried to give them model years but that goes away real fast. They may have a bus called a 2004 series or something like that indicating when it first went into production. However all buses will have a date of final production on a name plate somewhere.

In most cases bus delivery contracts to transit systems usually span many years as all transit buses are built to order."

So, if they have a transit bus called a 1990 series, does it more likely mean they entered service in 1989 or 1990?
a) 1989
b) 1990

When you answer, can you give me an explanation?

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