The side windows used on Vintage Boler trailers are known as Jalousie Windows. Although they may not look modern they are in fact a very efficient design for two main reasons.
First the window fully opens; sliding windows including the modern radius corner windows only half opens.
Secondly the way the window opens, as a awning, naturally sheds the water so they can be left open and in most cases rain will not get in.
I recently had the opportunity to rebuilt the Jalousie windows in my Boler to replace all the seals and gaskets and to polish all the aluminum since I had taken them out while the Boler was being repainted.
Disassembly of the windows is quite straight forward, the following video demonstrates how to disassemble, please excuse my poor acting.