It's funny, on my SL I had to go to the hardware store and pick up a bunch of different screws to test out to fine the right ones - on mine at least the back plate mount uses threaded holes, I was able to find some stainless screws that fit (was not a metric size as I recall), but only after some trial and error.

Sorry mine does not have a front plate, didn't come with one and I can't bring myself to drill holes in the pristine front bumper to add one.....

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1999 SL500