Jeff,
I rebuilt my rear calipers a few months ago. The 113 rebuild kit didn't fit, and eventually a local MB parts guy figured out that the calipers are 250/8 sedan calipers. Rather than hunt down 113 calipers, I just rebuilt and reinstalled them. They bolt right up and seem to work fine, although the pipe appears to have been bent to connect properly. I know the piston diameters were different, but can't recall now if the 250/8 pistons are larger or smaller than the 113 pistons.
George Davis
'69 280 SL Euro manual