Black Powder Express. That's probably the name givenby the mould manufacturer, likely Lyman. It would be a variable cavity mould, Here's an example of one in 350 grains from an expired auction
http://www.auctionarms.com/search/displayitem.cfm?itemnum=7465765&aa=%20%20%2035 0%20Grain%2050%20Caliber%20Rifle%20Bullets