Product review (4 stars): Same as Aprilaire 4040 except for intake filter
I bought this from another website where it was listed as Aprilaire 4040, but I received the Generalaire 4040. They say it is the same company. The only difference seems to be the intake filter. The part number on the valves are the same. The part number on the valves are also the same. I have included pictures.

The small Aprilaire filter on the intake side is black and made of plastic. The one which came with the Generalaire is gold colored and made of tiny balls fused together. My inflow pipe did not fit, because this gold filter has a taller rim. I could have cut the pipe shorter or filed it shorter, but I replaced the gold filter with the old plastic filter from Aprilaire which has a shorter rim and it fit again. Measuring the inside dimensions of the intake side of the valve, it is exactly the same for the Aprilaire 4040 and Generalaire 4040.

Make sure you turn off the circuit breaker to the heater before doing any work. My Aprilaire 600 is attached to the right side of the duct so it is actually  upside down. The pipe on the valve orignally was on the left side of the valve and did not fit the screw holes because there was not enough clearance between the screw holes and the duct the humidifier is attached to.  I had the installation instructions from the Aprilaire 600 and everything seemed correct. I called the filter store I bought it from and he was not able to help. I finally contacted the Generalaire customer support who said that this was not the right part and I needed filter GA35. I had to explain that this was a solenoid valve and she finally connected with the technician who was able to help me. He said the bracket can be rotated to the other side. I loosened the large nut at the end of the valve and rotated the outer casing so the foot bracket rotates around to the other side. There are no instructions with the Generalaire 4040.  I just which they had included a small insert about rotating the casing if needed.

I have included some pictures above.