Because I'm cowardly that way.
So, two examples of the type - narrow tang, widening out an inch or so to maximum depth of blade, before tapering down again. Usually pretty short, always narrow. Usually 1/8in. or so, but the bottom one in this case is a bare 1/16in. That, my friends, is mega narrow for any chisel.
I've never thought of them as narrow bench chisels, simply because my old man has a 1/8in. chisel by Mr Marples and his boys, and it's nothing like that. I've always sorta assumed they're a variety of mortise chisel, possibly for some specific purpose (not a rare one, because they turn up frequently enough), but having found myself in possession of that ultra narrow one, I'm rather more uncertain of their use than before. Cursory inspection of Salaman and an old Marples catalogue has provided no enlightenment. I'm not saying the answer isn't out there, just that presently I don't have the spare brain power to go looking for it. So now is your opportunity to shine, gentle reader - what sayeth you? The comments box is open.
Meantime, I may have just succumbed to the joys of secondhand books via the interweb (again) and another volume of the Woodworker for my, um, collection. All being well, 1933 should this way come shortly. That'll keep me busy...