Wow, blue grips, a low serial number, and only arrived today. Somebody's living right!
I'm not quite sure, but I believe that the "R" designation on the serial number was not present in the early guns, so it is possible, I would imagine, that there is a serial# 157 (without the "R" prefix) already in existence.
As to the blue grips, this is only speculation. I would imagine it is possible that your R9 was partially assembled with the blue grips, then set aside for some period until a time when the blue grips were gone and the black ones began to be used. Again, without a confirmation, this is only speculation.
Either way, consider yourself lucky - that's a beauty!