It's quite a milestone, as it's only the 4th class of double-deckers to make it into the 400s in the last 70 years!

The original Rs went up to R833, then there was a gap, and R901-926.

Next was the D-class, which went up to 840.

Then the Olympians, and now the AVs.

Large fleets of single-decks were even more rare with CIE, and to my knowledge the SS-class was the only one to reach the 400s.

Aquavid