...this feature was always on the early AMT model kits, overseen by Jeffries, which were based on the small studio model besides the 11-footer.
Beat me to it. I recall a little button on lower sensor dome of the 18-inch AMT model.
I think I can kind of see it here
but who knows if that's the original equipment. The main deflector dish and the nacelle domes were replaced, so maybe this was too.
Edit: this looks definitive: