We don't know what brand or model you put in, but
R = Registered or Buffered (usually seen only in Error Correcting (ECC) RAM
U = Unbuffered or standard RAM (Could be ECC or non-ECC, although Non-ECC is standard for almost all desktop and consumer machines)
Count the number of RAM chips on the module. If it is a multiple of 8, then it is non-ECC.
But if it is a multiple of 9, then it is ECC RAM and not appropriate for your Mac. If it says Reg. or Buffered anywhere on the label, it is also not for the Mac.
Thanks
Trevor
CanadaRAM.com