Here is a 'KISS' principle method of working out class without the complexity of the Aust.-dogs-breakfast-Class-System:

A $20k race is classier than a $15k race and will attract classier horses. A $100k race is classier than a $70k race and will attract classier horses. Get the idea?

A quick look at what class [how much was the race worth?] races were won by any runner in a race, lets you know what class of horse a runner is.
If it's had 6 career wins and the best it could do was win a $20k race, it ain't got a hope in hell of wining the $70k race it's in today [regardless of weight]!