Mystery shape on 1/50 DFM DC-9 decal sheet

[Henry Tenby]

Can anyone tell me what this mystery decal is that was included on a 1/50 DFM KLM DC-9 decal sheet (see photo below).

I have looked at many DC-9 photos and have studied the numerous colour images in Terry Waddington's DC-9 book and nowhere could I see how this decal is applied to the DC-9.

At first I thought it was a silver leading edge detail decoration but there is no such detail on any DC-9 photo I could find. And then I thought it might be a belly detail or over-rotation protector for the rear belly section of the aircraft, but again I could not see this on any photo of any DC-9 aircraft.

If anyone has the answer please let me know!