A billing department, some time in the 1950s, grew tired of typing addresses twice, once on the letter, once on the envelope, and cut a rectangle into the envelope's face, backed it with a translucent film, and aligned it so the recipient's address, already printed on the folded letter inside, would show through in exactly the right place. It was, for its era, a small miracle of reduced labor, and it has not meaningfully changed since.
Its entire function rests on a single fold, made correctly. Fold the letter one crease too high and the address slides out of frame, greeting the postal worker with a blank strip of paper instead; the specimen is unforgiving of imprecision in a way most office objects have stopped bothering to be. When the fold is right, though, the effect is a kind of quiet elegance: two pieces of paper, cut and creased by different machines at different times, meeting inside a sealed envelope to form one legible address, visible only through this one deliberate gap.
The film across the window has grown thinner and cheaper across the decades, prone now to static cling and the occasional tear, but the geometry underneath, window, fold, address, alignment, hasn't needed revision in seventy years. Somewhere in that unbroken math is a kind of fidelity most correspondence, digital or otherwise, no longer bothers to promise.