If you want to update your rims for a better selection of replacement tires, try to find some early W108 (66-68 250-280 S-SE) 14" rims. These have the three 'nubs' stamped into the center to fit your small center-caps. However, you would need to get trim-rings like the early Pagoda SL or W108 as I don't think there are slotted ones like your 13s availible in a 14" size.

Happy Motoring, Mark

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