Holy crap. The condition is absolutely spectacular. And personally I like the blood red over the blue of one that just sold for $5,600. Don't get too excited as that price is an anomaly. But then again, you never know.

As miamijim pointed out, your best bet for a date is on the lock nuts of the hubs. Otherwise, the best you could do is 1961-late 60s. Not many Bs were sold after the late 60s.
Is the RD Gran Sport, Record or Nouvo Record? GS=1961-1963. R=1963-1967. NR=1967+