If you are looking at posts from the bottom up, read the post above first so this makes a little more sense.
There were two of these pane-less windows in the wall of the Observatory, made by merely leaving (and/or cutting) a gap in the rock. They each point at one of the two “Sisters” mountains which can be seen to the east. Each window has wording etched in the rock below it indicating which mountain you are looking at and how far away it is. Pretty clever, even for 1935.
The mountain in the lower photo is North Sister. The window with the sign points to Middle Sister (but that sign was more legible).