If the "extra" pin is labeled "87" then it is safe to cut off. That number ("87") is German for "relay contact." It's probably defined as part of the DIN standard. Since the OVP in your car is warm and dry inside the passenger compartment, you don't even have to seal off the hole.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970