I had two spare keys made at an actual locksmith here in town, so not a Lowe's or Home Depot. Think I payed 8 dollars for 2 keys. They had no problems copying them. I have heard some horror stories about using such keys on a daily basis though. Apparently the copied keys are generally of a softer type metal that will wear down in the ignition key and leave metal fillings behind that can gunk up the ignition and give problems.

I made the spare keys just in case I lose the original so that I can still get in and move the car, but I will get a copy from the Mercedes dealer right away if I ever lose the original.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)