These are mounts for Side Boosters on a Model Rocket.

These can be mounted for show, or for use. The booster tubes are for the 18mm BT20 and mounted to a 56mm BT70. They are for the 18 mm engines. Either Estes BlackPowder ABC or Q-Jet C/D. If you just use boosters or want the engines ejected, Do not create a hole into the BT70 for the Cowling. If you want the boosters to provide / help ejection, you can make a hole into the main BT70.

Enjoy.