The polished stainless one is a replacement cap with dip stick.
The black plastic one is a tool for use on the standard cap.
Here’s the google translation of the black tool:
“Anyone who has ever tried to open the "dipstick" with their bare hands has more than cursed the system.
Since it is almost impossible to unscrew the oil dipstick without tools and scratches on a Norton 961, we have developed a tool to simplify this.
The tool is made of polyamide to prevent damage to the dipstick. When placing an order, care must be taken that Norton incorporates various designs of the dipstick. Therefore, it is essential to send a picture of your dipstick when ordering.”