I picked this up today in a local antiques center and though I knew of their existence, I had never seen another in the hand or for sale, so I could not resist buying. Used to drop messages to your allies, mostly for reporting enemy troop locations so your artillery can shell them. It was labelled RFC by the seller but I assume they were used in the twenties and thirties as well, so it might be RAF. The condition is fantastic and as far as I can tell the construction and materials are all correct for the period. It has a little pouch at one end which closes with a press stud and I assume you placed the message in there along with a weight and threw it over the side.