I don’t know about the P-51, but the P-38 doesn’t have a carb heat control in the cockpit on the grace that the turbocharger heats the air due to compression and the remedy for carb icing is to push the throttle far enough to engage the turbos.
If the P-51 has an automatic second-stage boost control, this could explain the lack of a carb heat switch-- you just have to kick in the second-stage supercharger to get carb heat.
Two possibilities when you encounter a piston-engined plane with no carb heat: It’s boosted, or it’s fuel-injected. One of these comes built-in with carb heat, the other doesn’t have a carb in the first place.