Quoted from jgelman:Good question. When I received the machine, it had that back door. I have no idea why it looks like that but clearly it was built to look that way on purpose. Maybe an EM historian can chime in.
I have no problem swapping it with the smooth back door of one of my other EM machines of the era such as my Buckaroo or Kings and Queens.
Interesting that I had several inquiries about my Flip a Card. That one is not for sale. Really my personal favorite EMs are Aquarius and Flip a Card even though those are not the classic collector Gottlieb machines like KOD or Buckaroo or Kings and Queens.
I imagine it may have been sprayed or coated with a material for noise reduction. The metal doors only do so much when it comes to sound reduction on EMs.