I can't remember the africa stone event, only thing that i can remember are the atlast stones, which are loaded on platforms, or different carrying events which seem to differ from contest to contest, and they carry things like shields (made from a slab of rock) and stuff - and that carrying event was 50 minute rounds x2 and it was for time.
But no doubt there is an event where they carry a stone for distance/speed as well! Strongman events are so versatile.
If they bear any resemblance to the Atlas stones they should be between 110-150kg.
Though a stone is much harder to lift than a sandbag, so its not really comparing like with like. Stones are slippery, and the roundness make them hard to grip at all. I'd think it is twice as hard to lift a round stone than a sandbag, so once the 220kg sandbag starts feel ok, then you are ready