On the brush roll motor, there is a three finned drive cog that sits in the brush roll. On DC24's, two types were fitted. They modified the three finned cog and the brush roll. Most machines are the later type as it was an early modification.
Only the later type of brush roll motor is available.
So if you buy one and it doesn't fit your brush roll, you do one of three things: change the brush roll too, change the three finned cog over (detaches with a T8 screw) or you can also drill down the brush roll a bit so the newer cog will sit in there.