His memories and tattoos - how could they not know?

It is has been exhaustively discussed how the physical appearance of the imposter seems to great for the family not to have known he was not NB... but what about the tattoos? The film shows what is apparently the missing persons flyer that the imposter got his hands on in Spain. It says he has a 'T' on his hand between his thumb and forefinger, a "J" on his shoulder and a "L" and N" outside of his left ankle. How could the imposter have replicated these marks perfectly with just this description to go on? Surely the family, his friends and the authorities should have noticed this? Also, it is never explained how he apparently has complete amnesia about the first 13 years of his life. That is not something trauma would do, what about everything he learnt in school, all the experiences he had with his friends, how could anyone believe all this information has just vanished?