The "stripes" were added in the early 90's. The first car I recall seeing them on was the AMG 3.2 W201, but it could have been debut'd on another model.
I was chatting with some amg guy on the Private Lounge and he said all you got in the early "70" +- (when they were working at the Old Mill in Burgstall, till they moved to Affalterbach, Germany in "78") all you got was the AMG with no line under the AMG and it was only on the front air dam. Not on any other part that they made. They didn't even have a trunk badge, which I badge myself cuz I found one of the underlined badges in Amsterdam (via ebay) and think it looks good.
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PS. I can only hope your sl's c's are driving as nice as mine is
Ah that magic feeling of being post mortgage and zero debt. (between repairs)