According to this web-site -> http://www.flippers.be/basics/101-interlock_switch.html, the Interlock switch was introduced "somewhere during the production run of the Twilight Zone pinball machine". Is it possible that my machine is an early production model without an Interlock circuit?  I have never seen the message "Coin door is open Coils and Flashers are disabled" on the DMD when the coin door is open, and according to that same web-site, "You can also see it's installed, the difference is that now there are 2 switches left inside the cabinet that the coin door closes. Older games only had one switch.". My machine only has 1 switch 

Matthew