your SM 23 did not have the rubber flap mounted on a stainless steel cylinder box.
This system came later probably around hull 100.
If your system has not been modified since original, your exhaust through hull system is just a GRP tube bonded to the hull, where the exhaust hose is glued into with silicone filler.
The rubber flap system has been installed on SMs (since hull 100) and on AMEL 54, 55, 64, 50 and 60 (not Santorins).
The plastic mufflers have been installed on SMs with Yanmar engines, all the Perkins and Volvo engines have (originally) stainless steel mufflers.
Mike, if you want to install the rubber flap system, you need to make sure the GRP through hull tube is the same diameter as the bigger diameter of the stainless steel cylinder.
Hope the attached picture will help and show those who have never seen this part (although they have it).