The hubcaps you are tying to fit were used on only the earliest 450 SL's ('73-'75??).
Your rims were used on most of the 450 SL's and all 380 SL's.
The newer rims, I think they are called 15 hole rims, were used on 560 SL's.
When people want to upgrade or modernize the look of the rims on their 450 or 380 SL, they always intall these 15 hole rims.
I'm sure they are sold on
www.thetirerack.com