Almost all open cars suffer from backdraught to some degree. You can see the effect with the dirt on the back window of an estate car or even on the number plate of a saloon. The abrupt end of the bodywork, and no one could accuse a Morgan of being streamlined, causes an abrupt change in direction of the air flowing over the body and a flow forward. You can physically feel this effect on a motorbike.
From what Gambalunga posted it looks like this backdraught is combined with a poor catalyst letting quantities of unburnt hydrocarbons out of the exhaust. This is what you will smell. Whether the catalyst also is letting out carbon monoxide, which seems likely, I do not know. Carbon monoxide doesnt smell .
Side exhausts will have a shorter hotter run letting the catalyst work better and could well be letting the gasses out into an airflow ehere they will be well diluted before any backdraught at the rear of the car.
