As far as I know, all the 450's were white, and the 451's were yellow. I may be wrong about that, if so, I will stand corrected.
If no other way, you can tell by the ratio. The difference is in the gearboxes. The 450's ratio input to output is 1-.54 (1.56-1). The 451's ratio is 1-.39 (2.59-1) so you can just spin the input, and check blower rotations. If you put the 450 on your tractor, at 3600 RPM (WOT) you will overspeed the gearbox by about 400 RPM. Your choice. If it were me, and you have both available, I would go with the 451. But, you could run the other, just either: Don't run WOT. Or, be prepared for the gearbox to fail. If it was just 100 RPM, you'd probably be fine. But your looking at a 33% increase in speed at WOT. That's quite a bit for that gearbox. Hope this helps!
