Description of window set, in case someone knows them by make: the lower pane is counterbalanced (weight? spring? I don't know) and the upper pane is fixed in place with a screen window underneath it. Each window frame also has a pair of lightweight, removable inner windows that can slide past each other. So that's three sets of rails for sliding windows, plus the upper window & screen on the exterior. The screens have one knife latch on each side, towards the top.
I took some pictures, with annotations.
When unlatched, the screens are free to slide around by about 1/4", but are completely trapped between a metal frame on the outside, and between the lower pane & a metal rim on the inside.
- interior: the screen is trapped by a metal rim at the bottom (about 1/2") and the raisable window at the top. When the window is opened to its highest extent (touching the top of the frame) the pane blocks the screen by several inches.
- exterior: the screen is trapped by metal rims on all four sides, by about 1/4" to 1/2".
Upon further inspection, I think the screens are supposed to pop outward, but I don't see any removable parts in the frame, except a few screws on the outside. Removing them might be the answer (or a red herring), but they're thoroughly seized in the aluminum frame.