Obviously the single outlet version makes no attempt to copy the original and the original appearance will be lost. Assuming the single pipe is of sufficient diameter and the rest of the muffler is designed and manufactured properly it should be OK. If it is considerably different to the original there can be no guarantee as to how it might sound though. All the W124 wagons used mufflers with a single, large diameter outlet. The four cylinder sedans (200, 200E, 230E, 220E & E220) also used single oulet mufflers, however they would be unsuitable for the 300E as they used a smaller diameter pipe.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.