Nudge Bar

Hello all, I do a lot of vermin control and have just bought a nudge bar for my 56 Freelander Commercial to enable me to fit some spotlights. My problem is that I bought it from Formula 4 and they haven't included the fitting instructions and it goes in on Friday for the lights etc. Would anyone have any idea what the detachable stainless steel plate is on the middle bar and what the black steel bar on the top of the frame fits to. Any help would be great. Thanks in anticipation