This is a multi piece fin bracket for water rockets. They allow the fins to be bolted in place and firmly tightened to fit around the rocket body, but can be easily removed at the same time. The file provided must be printed 6 times in order to have enough pieces. You will also need 6 M3x8 bolts (Preferably nylon for safety) to hold the fins on.
Please note that these brackets are designed for UK spec 2 litre soft drink bottles. These may not work with other diameter bottles although small variations in size may not matter.
You will need to make your own fins. Suggested materials are corflute or balsa wood. These will require 3mm holes drilled at certain intervals as well as a small ridge cut for the bracket to sit nicely (see picture).
Skip to 9:12 in the video to see more information on the fin brackets.