Walther lg400 Alutec hand stop

I recently purchased an LG400 Alutec for my son who competes 10m 3 discipline. He needed a hand stop so I ordered him a TEC-HRO hand stop. It doesn't appear to be the correct fit for the bottom rail. Seems to be no resources as to what hand stop is the proper fit??? I purchased it from Creedmore and still awaiting an answer as to whether it is supposed to fit or not. Any info would be greatly appreciated.