As I've always read, the cuffs were there, not for the radiator, but to improve the airflow into the carburetor intake. I do know that in other past discussions on the topic, current pilots that have flown Mustangs with the various different props and prop configurations have stated that they haven't noticed a notable difference in airspeed performance between having the cuffed-blades and having the non-cuffed blade types (though I assume all in relation to regular "non-racing" operation).
During original manufacture, all P-51B/C's and most P-51D's were manufactured with the cuffed Hamilton Standard blades. The K's were of course manufactured with Aeroproducts blades that didn't have cuffs. Late production D-models from Dallas (D-25-NT's, D-30-NT's) and some of the last D-30-NA's from Inglewood were manufactured with the 'paddle-blade/square-tip/cuff-less' Hamilton Standard blades. So at some point it must have been found that the cuffs weren't really all that beneficiary/important to have.
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