The so called 'blades' were not factory installed. Back in the day, cheating customers would attempt to bore a small hole into the cabinet (somewhere just under the wooden side rail ) using a pocket pen knife or something similar. Once the hole was made, they would insert a piece of a metal coat hanger and trip various targets and rollovers in the attempt to score enough points to win free games.
The inside metal blades were an after market product sold by parts companies to Operators to stop the cheating. Some Operators even made there own blades cut from different types of metals and installed them.