The markings you are referring to on the Black Knight playfield were present and factory manufactured when Williams had the playfields made.
It wasn't that something was removed, it was that something was never installed in the first place. That unpainted wood space was intended to be cut out of the playfield and have a leaf switch installed in the horseshoe shaped gap. The white rubber would hit the switch and awards 50 points on a game like Algar where a leaf switch was installed in the factory behind each of the drop target banks. Looks like games prior to Fall of 1980 had them.
Games like Black Knight never had the switch installed.
Games that came after Black Knight such as Jungle Lord and Pharaoh, do not have those playfield markings like Black Knight does. It looks like they had originally intended to install that switch on BK, but then never did. Later games had no markings, so the intent to install that switch disappeared.
Games that came before Black Knight such as Algar, Gorgar, Firepower and Flash, all do have the leaf switch installed behind the drop targets wherever it was feasible.
Great question and I was fascinated, so I went and visually inspected these games this morning. Sample group today was 1 x BK, 1x JL, 1 x PH, 2 x Flash, 2 x Algar, 3 x FP, 3 x Gorgar.
