On my '73 350SLC they are both on the side towards the middle of the car. I don't know if it's a right-hand-drive versus left-hand-drive thing though. My right-hand-drive misses out on an armrest due to the central location of the hand operated parking brake (similar to the W201 190E). From memory, the left-hand-drive W107 got an armrest and foot operated parking brake (similar to W124 300E etc). Does the armrest require the left-hand seat to have its adjuster mounted on the left toward the outside?

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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.