Ferrari “introduced” the cut on the cover during 2004 preseason testing when they were using the shelf wing above gearbox on the F2004. Then in the last tests in Imola the wing (and the cut) disappeared and the airbox wing appeared instead. The shelf wing and relative cut were then adopted again on the F2004M and the F2005. Other teams adopts the solution with or without the shelf wing.
An interesting thing, notice that the rule says :
3.16 Upper bodywork :
3.16.1 With the exception of the opening described in Article 3.16.3 [hole to lift the car - Reca], when viewed from the side, the car must have bodywork in the area bounded by four lines. One vertical 1330 mm forward of the rear wheel centre line, one horizontal 550 mm above the reference plane, one horizontal 925 mm above the reference plane and one diagonal which intersects the 925 mm horizontal at a point 1000mm forward of the rear wheel centreline and the 550mm horizontal at the rear wheel centreline.
As you can see in the R25 the rule is respected to the letter and the engine cover has consequently that triangular end.
If you look at Mp4/20 on the contrary you’ll clearly see that the cut is larger than theoretically allowed, it goes above the horizontal line 550 mm above RP; the trick is that on the side view there’s still bodywork in that area thanks to the vertical fins between the wheels.
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