From Russell -- Braddock's tatoo:

the tatoo gave us great challenges,for although it was obvious he had one,it took a long time,digital technology and a stroke of luck to uncover.
I think it was the make-up guy,david anderson who actually solved it.
for the most part jim's tattoo looked like a boat.that didn't make sense to us,we couldn't understand why he would have chosen a boat.
we discovered a similar story re the getting of the tatoo*,however possibly a little older,post meeting mae.
the stroke of luck came in someone finally finding a photograph of braddock,sleeveless in daylight with the tatoo partially visible, david was able to magnify the image digitally.
we all fell in love with the discovery.
what we had thought might be sails,turned out to be the u.s flag,rudimentary without all the stars,the bottom border was flowers,at the centre? a profile of a woman baring a striking resemblance to his future wife! same hair,same lips.
our jimmy was a romantic soul.

*********************************************

*From a fan:

I have researched it and found out that when he was 16 some friends got him drunk and he had it done in the Bowery in NY. However, it did not say what it was. The shot I have seen from the film looks fairly specific as if it was covering a large area so there must be a story and as his family was involved in making the film, they must have known what it was.