Similar problem with my 260E. The MB parts guy was no help, I had to listen to his bs excuses for too long. Basically, the front screws go into a rubber insert in the bumper cover. They're self tapping screws that look like the cheapos that come with knock-down furniture. They're 2 mm dia by 1.5 mm pitch (very coarse) and (guess) 15 mm long. Not at all critical as they go into those rubber inserts.

He had no clue about the rears screws that go into the plate holder. I wound up taking the plate holder off, drilling out the holes and tapping for 1/4" x 28 x 1/2" long. I used stainless capscrews (bolts) and plactic washers over & under the plate. Be sure to not use too long a screw as it will hit the painted trunk surface.

The way he handed me the obvoiusly cheap screws with the offhand comment, "Here, take these." I thought he was giving me some junk screws (freebie). Then he called me back & handed me a bill for $5 for four junk screws that probably cost a penny total. Whatta guy.

BTW, I looked in the typical car parts stores, the license plate screws they have don't fit. If you have to use the right screws, remove the plate bracket (4 phillips screws) and take it into the store & try a few sizes. Get washers to cover the holes in the plate & not so long that they touch the painted surface under the bracket.

Enjoy,

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Norm in NJ
Next oil change at 230,000miles