As has already been said the scene clearly implies that it is a military tattoo.
I think I can explain further. The old guy is obviously ex military and of the 'seen and done everything in my time' variety as evidenced by his knowing everything about the out of date FBI card from a half glance. The other guys in the store respect him.
From his stunned reaction, and the effort they've put into showing that this old guy knows things ordinary people don't, we can assume that this is not any special forces tattoo but most probably one from a unit (fictional) which does not officially exist and few know about.
As for why the cards, if they are the worst hand you can get then I'd say its fairly obvious that it means the unit is regularly in extreme situations with no back up. They don't have an ace up their sleeve, they have the worst hand imaginable, meaning they are used to having no assistance of any sort because officially they are not even there so the whole operation is always deniable.