I switched my 87 300D over to an outside vented fender. I could probably text you a picture of the inside of the fender, or if you happen to be in my area, you could look at it. If you want the picture, just pm me the phone number to text it to.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes