Did you order it from Brownell's? Some where on the label of the Mono grip packaging it will state whether it's for a round but or square butt, and there will also be a part number or model number. Just give the folks at Brownell's a call and they'll help you figure it out, they're great folks.
If so, did you get the one that fits the butt of your pistol? I made that mistake not to far back, I accidentally ordered one for a square butt, but my revolver has the round butt, who knew? BTW, it doesn't have anything to do with the shape of the bottom of the grip frame, it's based on the contour of the back strap of the frame.
If the back of the of grip frame on the revolver is curved, it's probably a round butt. If it extends down with a straight appearance, in other words not curved, it's probably a square butt.
GS