Humidifiers and Dehumidifiers - Installing Aprilaire 700 - where to attach screws??

Has anyone actually installed one of the 700 model units? I need a silly question answered.

The instructions say to "Fasten base to duct using 4 sheet metal screws in holes provided."

I cannot find for the life of me 4 hole in the unit that would attach it to the sheet metal! There are two at the top that are barely going to touch at all, but I don't see where two more hole are that could possibly touch sheet metal.

It's a heavy fan unit and I don't want it to fall (I know the clips would make that practically impossible, but still). I want to mount it correctly but I don't see where they'd possibly want me to screw it in!

Edit - I installed it with three screws across the top, none at the bottom. It's not perfectly flush, so I'll caulk it to get a good seal and not lose all the heated air.

I've never put Aprilaire in, so I couldn't say how the screws has gone in.

I would put a couple in on the bottom to keep it snug up to the duct.

The base has clips along the bottom that go over the edge of the sheet metal. Then it is held in place with screws on the top.

I guess that's true, no screws along the bottom at all. Weird.

I would put a couple in on the bottom to keep it snug up to the duct.
I would but there's no place to do that. I'll just caulk it up for a seal.

Might seem weird at first but it makes sense since forces pulling the unit out of the plenum are concentrated along the top. Also came in handy for me since the unit is down near the floor and screwing in the bottom would have been a PITA. :)