Product review (3 stars): Tube too short for model 550
This will not fit an aprilaire model 550. The tube length is too short by about 2 inches. The fitment guide does not list the 550 but there appears no other kit for a 550. So this is not the fault of the vendor.

I did make it work. The model 550 has the water valve at the bottom of the humidifier case. The tube is too short to reach the top of the water tray. Also the tube must make a 90 degree turn when it enters the case which causes the tube to link and crack.

My solution was to move the water valve to the top sidewall of the case and then use a brass tube to go from the valve to a 90 degree elbow. Then the elbow connected straight to this tube inside the humidifier.  No bends and a simpler installation. I sealed the old  holes with silicone caulk and metallic tape. Works well.