Wow. Nice find. Back in the '60s I flew a BT-55 model on a MPC B3-3 motor. It got a foot off the rod before falling to the ground and ejecting after the delay.
Later I was an assistant for the NAR Standards and Testing Committee that tested the MPC Minijet motors.

Chas

__________________
Charles Russell, MSgt,USAF (ret.)
NAR 9790, Lvl 1
SAM "Balls Three"