Extra Part Appears During Reinstall

[BabyBlueBenz]

As I was replacing the Mixture Control Unit & Air Guide assembly back on my '86 560SL today, I spot this washer sitting atop my Throttle Valve Switch:



It's a washer. It looks recently used. I never saw this part before. I was replacing my injector insulators & seals and some of the idle air distributor hoses today, repeating the procedure I documented at The 560SL Throttle Valve Switch Replacement Narrative

After getting it all back together, I took the car for a test drive and nothing extraordinary happened. In fact, besides my ongoing high-idle symptoms, she's running very well.

Does anyone have an idea where this may have come from?