What is Anti-tamper device?
What does it do?
What is its part number?
How important to have it? How does this device effect car performance?

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1987 MB260E 187K

I'm guessing you mean the device that sits between the fuel distributor and the air flow sensor inlet. That's there so you don't mess with the fuel mixture setting. I've never done it (honest, I got my car that way) but supposedly you break off the anti-tamper device so you can tweak the mixture setting, then you're supposed to get a new anti-tamper device to seal it. I think you can leave the anti-tamper device off with no ill performance effect. Legal effects depend on where you live and who checks.

Sixto
95 S420
87 300SDL