Highly likely you will have to flash it if it is a new rebuilt dynamo as you say. Take out the connections on the F & D of the end cap and take a live wire from the negative - terminal of your battery, then just touch the F terminal on the end cap. One second is enough NO more! Reconnect the F & D wires to end cap and job is be complete. Often dynamos need re-flashing if they are stood unused for a long period of time as the very small residual magnetism fades.
Note: When flashing you will likely see a very small blue spark, this tells you flashing is done. If you should accidently use the positive + then no damage will have been done.