On modern English cards it sits in the bottom-left corner, next to the collector number, for example 25/198
The drawback is that creature has to have a mana cost equal to or lower than the number of lands you have, but thats super easy to get around in green
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Broken doesn't even begin to describe Skullclamp and it deserves to remain banned for all time
If this is compared to Consecrated Sphinx which generally gets at least two cards and is likely averaging about 5 per cast you can see why I don't think this is over powered
However, if it has an ability that triggers when a spell is cast, it will have been sacrificed before that ability can trigger