The small window screens are installed the same way as your other windows
except there is no seperate frame for the screen. The screen is mounted directly into the window frame. There should be a rubber bead that is interted into a groove in the frame, the sceen is placed over the window, overlapping the groove and the rubbber lockstrip (with the screen under it is then pushed into the groove, that holds the sceen in place.
When I rebuilt these windows
on my Boler
rather than replace the glass gasket I placed a bead of butyl tape around the inside of the window frame then installed the glass into the bead, this seal is working very well and should last a lifetime.