Lancelot is right, the 1.6, 1.8, and 2.0 were Willamettes while the 1.6A, 1.8A, and 2.0A were Northwoods.
He's also right about there being no 2.4A Northwoods. The 2.4A is a Prescott.
MadOnion, if you have a 533 bus 2.4 Northwood, then it's indeed a 2.4B.