"Plastic anti-vibration protector" also known as 'a washer'.
:-D
That A1000 box was incredible for it's age. I remember selling a PC Engine LT with and without a box and came to the conclusion that a small bit of cardboard with the right picture on it can be worth 150 USD!
Not sure I'd find an A1K that useful though, it's probably going to a museum... then again if people are still expanding their C64s...
;-)