Cluster centering ring, fits 4.0" airframe. (2) 29mm (1.1") holes. 3/16" birch plywood.
Fits into any 4" airframe. Works best with Aerotech paper, LOC paper, fiberglass, Semroc 29mm motor tube. (1.22 - 1.23" motor tube OD)

Please see next item for same ring cut for different 29mm tubes.