MP is Mather & Platt Ltd, Manchester - the company made both 3-inch and 4-inch Stokes bombs, and component parts of fuzes, amongst many other products.
MP was the monogram used by the company on larger items such as 4.5-inch shell, MR for Manchester.
MR
(May and Padmore Ltd, Birmingham, are a possibility for making Stokes pistols, but the monogram M&P usually appears on their products. The company definitely made brass plugs and exploder pockets for 2-inch mortar bombs, and these are found marked M&P.)