Product review (3 stars): Not worth saving $2
I bought these to use in an Aprilaire 500 unit, which this model claims to be compatible.
It might be compatible, but not a good fit.
Much thiner than it should be to fit in the frame. It was so loose that made me conerned about it's durability within the unit, it would probably wobble with the wind passing through, so I added two water bottle caps to make it stay secure in place.

I bought original water pads for the Aprilaire 700m downstairs and that was a perfect fit. IMHO was not worth the economy for this generic brand.
Next time I'll spend buying original parts.