Help - Can someone identify this piece?

[boxy1004]

Hi everyone,

I was lubricating my 987 convertible moving parts, track, arms etc when I heard something drop, I look down near the drain and find a black cap looking part. It's the circumference of a nickel, dimple on one end and hollow on other. Please see pic. For the life of me, I couldn't determine where it came out of?? Can anyone identify? Thanks in advance!

[Beelzy]

Cap for hardtop spin lock hole?