Surprisingly, they were installed post production by operators who didn't know how to change the settings to make the game easier. A simple task of changing dip switches and free game settings was too hard ! So destroy the playfield by installing a mini post. Having said that, the games were designed to make money, so there was no care in changing the game by installing the post.
Some games were released with the posts - Classic Sterns like Meteor had factory mimi posts.
