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A key is a blank (that has to fit) and then 5 or 6 "teeth" which usually have only 3 or 4 possible depths - that means you have only a few thousand actual possible combinations (some won't work because some locks can't handle two identical in a row, or a tall followed immediately by a short, etc.

Also as cars get older, the ignition and locks get looser until a screwdriver will start it.

The “stolen” one is a 1991 model according to the article, so not that new (just kept in immaculate condition as the photos show, I was surprised when I saw it was a ‘91), and only 6 years between the two vehicles involved. Given those ages, it’s not shocking Mercedes was using the same key patterns.

In fact other German cars are susceptible to this too. Famously VW uses so few combinations on their keys that it's actually not at all unlikely your key will work with another random VW of the same vintage.

They were '91 and '84 models.