If the mixture is over rich and there is no air injection, there is insufficient O2 in the exhaust to sustain a reaction in the converter, so I don't think any damage would be done. Since the early/mid eighties Merc engines have had TWCs (three way catalysts) and if there is an air pump it should only be active before the engine goes into closed loop.

Earlier engines (pre 02 sensor technology) with OC (oxdizing catalyst) had air injection at all times.

The type of catalyst is listed on the emission/tuneup decal in the engine compartment.

Duke