There's your answer, it's the squish head that helps prevent detonation! My experience with the standard hemispherical head (without any squish band) on a 150 engine was that it would occasionally kick back, even with a fully working advance mechanism. Mind you, I probably didn't help matters by using a skimmed head.....
I don't believe the Varitronic ignitions provide any substantial ignition retardation for starting, and only start to retard the ignition at engine speeds way above the normal operating band of a D/LD engine. On the other hand, the original Marelli or Filso ignitions would give a timing of around 8 degrees BTDC for starting, advancing to 26 degrees BTDC by the time the engine reached around 1,800 rpm, and remaining at that figure for all higher engine speeds. Although it provides breakerless ignition for D/LD engines, I fail to see why the Varitronic's ability to retard the ignition to prevent detonation above about 6,000 rpm is of benefit on an engine which maxes out below 5,000 rpm!