Product review (5 stars): Direct Replacement for Aprilaire 550 Solenoid Valve
The 4040 is the same part number referenced in the vendor literature for the Aprilaire 550. There are 2 images for the 4040 online, the part I received matches that shown on Amazon. Although my original solenoid valve is the other type, this was a perfect replacement. It interfaced to the existing plumbing on both sides. It also came with an inline strainer (the gold metallic type) installed on the intake side.

Installed today, functions as expected - No flow when the furnace is off and flow when it is running.