My thoughts exactly - if you are "restoring" it (insert large WINK here) then there is no copyright issue - if you are selling a new one, different story - I imagine they want to keep a fairly low profile so as to not attract the interest of the "keep pinball in the 30's" brigade.....
It seems that they dont do the mirror/silver parts of the back glass, they are instead done in grey.
Someone one aa, said they bought 4 glasses and did not have to sent any to be restored. Sounds like a copyright issue to sell repo glasses, just a restore/repair service.
