In my house, there are 3 under the bay window and one under the adjacent (joined by an arch) room in the main “posh” room. The other less posh rooms seem to have had ordinary folding shutters. I’ve found some of them in the loft (used as makeshift boarding) and some are still in situ but painted over. Maybe I’ll find some sliding ones in the loft . Next time I’m up there I’ll look a bit harder, among the junk (none of which is ours!) and several redundant water tanks