What was the 1st numbered model?

[tractorboy]

Can someone help me? I'm trying to find out which was the first 1-75 Series model to have a number put on it.
I was going to get my models out and look through but felt pretty sure someone here would know the answer.

[Idris]

The 24a was the first model to carry it's number on the underside/baseplate (although the wheels are numbered 23!). However, it could be argued that the 5a was the first model to carry it's series no. - albeit on the front and rear as the route no,!

[tractorboy]

Thanks for that, much appreciated.
I did read through the McGimpsey/Orr "Collecting Matchbox Diecast Toys" book and saw it mentioned the wheels but couldn't find any reference to base numbering.