I would guess it’s a version of the Fiske coffin. These coffins were designed after the advent of major railway transportation and offered airtight coffins. This helped to naturally preserve the body. When people began traveling far from families and then subsequently dying, people needed a way to transport bodies long distances to get them back for the funeral and last goodbyes from living relatives.

The window was put in place for health reasons. Should a person have died while in quarantine or from a deadly disease, the window allowed relatives to see and confirm the body's identity before burial.