Fullmetal Alchemist: Brotherhood

[beyondthehorizon]

That arm of creation, or whatever he called it while fighting Bradly. Did that essentially come out of nowhere, since I don't recall ever seeing those transmutation tattoos at any point before then? Or is there some plot device that explains that away?

[RubberLuffy]

Scar had his brother's notes, and his brother had the same tattoo.  http://fma.wikia.com/wiki/Filecar%27s_Brother_Manga.jpg

[beyondthehorizon]

So Scar basically gave himself the second tattoo at a later point?

[RubberLuffy]

Most likely during the timeskip before the promised day, since they had the notes at that point and were using them.