Part of the ruling announced today regarding registration in the US beginning on December 21 pertains to marking our P3s. You can use the P3 serial number if you choose, or use the FAA issued registration number. One good thing I see today that I didn't see in the initial recommendation last month is that marking inside of the battery compartment is considered "readily accessible." If you are in the US and you decide to comply with the registration and marking regulations, where are you planning to mark your P3? Somewhere on the outer shell? If so, where? In the battery compartment? Update - the snippet above was taken from the DOT/FAA final ruling document published today, but its from a part of today's document that was providing a summary of the recent task force recommendations. It seems that later in the document it is clearer that the recommendation of using your P3's serial number was discarded in the final ruling.