Movies and TV shows shot in 4:3 but shown letterboxed.
I don’t understand how you can letterbox something that is either the same ratio as the screen or taller, unless you are windowboxing, which is different.
…unless you mean letterbox on the sides rather than the top and bottom.
This is usually called pillarbox, so saying letterbox and meaning pillarbox is misleading.
I just looked up the terms. Windowboxing is when an already letterboxed image gets pillarboxed. An example we should all be familiar with is the GOUT.