As mentioned in my Run-Off Groove post, this was the mysterious inside card gasket Ferryman found in his JR21 New Flying Saucer way back in 2009 here.
The mystery persisted for two years until I chanced upon the answer, a toy box, as detailed in Gasket Case back in 2011.
As we approach our 7th [or 8th?] birthday next month, I forget just how many toy mysteries have been solved since we started back in 2008!
Can you think of any mysteries left readers?