Well spotted :-)
it reads: Made in Germany.

the plastic cap is screwed on a threaded pin, sticking out the bottom of the trunk
I guess securing a spare wheel is the most logical purpose? interesting when you have this in a car with 2 different rim sizes :-)

the top of the cap is missing something, which should be kept in place by the velcros
looking at it a bit closer; it also has "Mercedes Benz" "SP 3307" and "A170 890 03 14" written on it
The opposite site of the cap has a table printed/pressed on it...
I'll try to get this on a photo.......

Tx Tom