Joe,
Would you have a slide over piece that we can tighten down? One that will fit up to the back of the original notch but not pass it? Its important that the lower notch not be changed. Nor the newer piece set back farther past the original notches fixed position.
The reason for this is each of our 500a models holds a tactical 37mm launcher on its underbelly. Less lethal munitions such as CS gas and Hornets nest rubber grenades are the perfect munitions for fugitive recovery. The forend pump grip has been modified and extended four inches forward with a weaver rail to facilitate the extra length and weight of the launchers. The extended tube is steel and has a fitted extended slide mechanism for the loner forend so things stay tight and not floppy on the tube. when the grip is in its regular firing position it rests only about a sixteenth of an inch from the mount. Wouldn't want to chance the custom work impeding the movement.