How was Davos identified, by his shortened fingers or by his head? I don't believe his face was so known in the White Harbor (specially to the Freys who were one of those who identified him) so his shortened fingers were the answer. Most commonly Manderley and Davos put some bad guys head on pike, so why not his hands? Point is, how many men with shortened (left) hand fingers runned around in Westeros? Was Stannis punishment common punishment to the smugglers or act of mercy? Fact is, they cut off all the finger of thief (Tarly in Maidenpool), not just a tip of it. Were the smugglers lighter category than thieves in Westeros? I don't believe so. Most probably Stannis with all his great and kind heart didn't want to do much damage to his Onion Knight and cut off his fingertips not all fingers. Probaly was Davos with his shortened fingers maybe even as rare as white craven in Westeros and White Harbor dungeons didn't swarm with rascals with shortened left hand fingers. So what to do when you have somebody whose head you can cut off, but whose fingers don't answer to the parameters. Cut off his fingertips of course! But... there is very big difference between freshly cut fingers, still raw and dripping blood and old cut what was made years ago. So I believe that Manderley and Davos just run short with shortened fingers and Davos did his heroic deed.