The pin that came with the brake isn't original...it is longer for the brake. The original was discarded in lieu of it.
You may want to hunt down an original if the long one can't be recovered...you do need both pins. It's a hoot shortening the lil buggers when you need to...be ready to cuss...have precision vise grips, and don't forget eye protection.
In a pinch a roll pin can be subbed temporarily if you wanna send the original pin for the brake along with it.
I have replaced a FSB a couple times, both pins can be crucial to maintaining straightness. I will say that you having the likely unfettered with original FSB will make it easier to fix, and less likely to cause issues...but you should put A
pin in there...regardless.