Hi Iwik,
Some think this cap was a fuel filler, but the pipe runs down inside the right side of the cockpit then forwards and across to the lower rear of the engine. The pipe seems to be attached to the oil pump housing, so I believe this pipe was the engine crankcase breather pipe, to relive excess air pressure built up inside the engine.
The pipe into which the breather cap is fitted was made from brass micro-tube. The breather cap (kit item C9) is molded as a solid piece, whereas the actual cap had breather holes around the top of the cap.
To replicate this cap I cut away the stem from the kit part and drilled a 0.7 mm diameter hole up into the cap body. Into this I secured a short length of 0.6 mm micro-tube using CA adhesive. The cap was then drilled around the top edge with a 0.3 mm diameter drill to recreate the vent holes. The cap was brush painted with ‘Mr. Metal Colour’ Stainless Steel (213). I then inserted the new stem into the fuselage pipe and secured it in position using thin CA adhesive.
Mike