Always verify the legitimacy of individuals or organizations before sharing personal info or making payments. Do your research. RSA is not responsible for losses or damages caused by scams or fraudulent activity.
I don't know if the U has any special significance, by Remington used a U to mark some of the parts on Civil War rifles and muskets. Not all Rems have been marked with a U, so I'll just post this guess, and hope the real experts jump in and educate all of us.
Dan