It looks to me like you have to tap the hole in the MRod trigger group to accept a screw to hold the Challenger guard.... I presume on the Challenger there is a screw that passes through both and screws into the main tube.... You may also be able to countersink the trigger block from the back and use a nut and bolt to fasten the guard in place.... Sorry to be vague, but I haven't mounted mine yet.... There is definitely a hole in the MRod trigger block that lines up with the mounting hole for the Challenger guard....Bob
I can inlet your stock for a MRod trigger guard and supply you with a MRod trigger guard PM me if you are intrestedNormDiscos R Us
Norm did to my disco stock what he offered you with the mrod guard and you can see how it turned out from the pic in my sig. Looks great.
I tapped out that front screw for the guard .