The thumb cut-out, while on a right handed rifle is for use with stripper clips, they are just the bees knees for us left handers when loading the magazine, especially under a scope. Much easier to position a big round on top of the follower and push down into the magazine using the thumb cut-out. Especially so on a Mauser where as xausa points out, the stock is cut away to fully expose the thumb cut-out.