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'm 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?

It was designed and first built during the 1990s, and the manufacture date is what would be stated on the title, not the series. The series may still be the current manufactured model for transit buses built last year. That series would not have been delivered until after 1990.

Impossible to say. You know that was 25 years ago, right?

It doesn't mean anything more than an inventory number so they can track expenses and income and maintenance records for that particular bus. It's easier than using the 17-digit VIN#.