I'm going to guess they're #8 round head sheet metal screws. Yes, they do go in from underneath. The defrost nozzles (I refer to the hoses as ducts) have a slot on one end, so you can start one screw, fit the nozzle, then install the other screw. Probably 1/2" or less in length. It's been a long time since I took mine apart, so going from memory.
