Sorry, I don't know if the W116 tumbler will fit. As to the tumbler vs key being bad you'll have to experiment a bit and take your best shot. If you have the extra key or valet key and it works normally then I'd guess it's 90+% probability that it's a worn key. If you don't have a spare key then it's much harder to tell.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold