Stihl Fuel cap .
00003500533 , 0000 350 0533.
This is the more modern fuel filler cap for many models .
MS181 , MS211 , MS231 , MS251 and many more .
This cap flips up to clip in and out .
This cap is for the fuel tank , if you require the cap for the Oil Tank the part number is 0000 350 0537.8.40