I never ever saw any Brass Buttplates, & to my Knowlege none were ever issued! There WERE, however. Two variants of buttplate in Brit/commonwealth Service. First pattern had a 'Trap' that the used could open & store the Oil bottle, & pullthrough & combined weight inside the Butt. The combi-Tool, debris brush & bronze chamber brush had to carried in personal equipment or in the smock somewhere.
The Second Pattern, was blank with NO provision for a trap to allow access to store anything! If I remember correctly, there was an E.M.E.R mod that entailed the gluing shut of Buttplate Traps when the stock of trap springs ran out. As the cleaning kit was for a LONG time issued in a Tin, & later a plastic container. this really rendered the old 'store inside the Butt' method, obsolete!
When later on the Full Plastic Black Furniture came into Service. There was NO provision right from the start for the user to be able to store anything inside Butts whatsoever.