Hi Btns,
Not going to be much help I am afraid.
They are not really my collecting area, so this a bit of guesswork & assumptions.
I had assumed they were worn by Officer's on their Mess Dress Vests in 'moneyed' Regiments, based on the fact that I have only seen them for the Cavalry & Guards Regiments. But that doesn't mean there are not any similar Infantry/Corps Mother of Pearl buttons out there - just that I haven't seen them or paid attention to them
There is a similar Mother of Pearl button for the Coldstream Guards on eBay at the moment.
https://www.ebay.co.uk/itm/WW2-Colds...QAAOSwfCZciCCI
Sorry couldn't be more help.
Ian