Product review (2 stars): Don't believe the sellers hype.
Sellers info appears to be incorrect. This #10 unit does properly fit my AprilAire 550. But the seller states it is 50% more efficient than the standard metal/clay pads -- NOT TRUE!  The box says it uses 50% less water than metal/clay pads.  50% less water equates to double the efficiency -- NOT MY EXPERIENCE EITHER!  After days of running at max. humidity level in a properly tuned humidifer & furance my water usage is still 3 GPH, same as the metal/clay pads.  Did the humidity level increase - yes, sort of by 1% from 27% to 28%.  I live in Illinois so the furance runs a lot during the winter.

Part of my problem which I hoped to correct with these pads is that when the 550 was installed it was added to my gas furance, it humidity levels could reach upto 45% in the winter. It is connected to the hot water line too.  When our A/C went out we replaced it with a Heat Pump which I understand can reduce humidifer efficiency by as much as 60% in winter.

These units fit a model 550 & they work, but no better than metal/clay pads. They need to be changed twice as often & cost twice as much.  Save your money & stick with the metal/clay pads instead of these paper pads.