Throughout the decades flipper pinball has existed, numerous designers experimented with innovative concepts, goals, and even how the game itself was played. One of the strangest and unique layout decisions was the use of ‘middle-pop’ designs.
What is Middle-Pop Pinball?
There is no formal definition for what is or is not a middle-pop game. At its core the term implies “a pop bumper between the flippers”. The concept was defined by hobbyists rather than the industry, and the pinball hobbyist community has an entire thread on Pinside dedicated to discussing middle-pop games. The participants in that thread often have differing views on what may or may not constitute a true middle-pop layout.
For the purposes of this article, the author has chosen the following criteria to define a middle-pop pinball machine:

The game must have at least two flippers.

A pop bumper must exist between the lowest two flippers, and this pop bumper must:

Be horizontally centered between the lowest two flippers.

Can occupy a range of vertical positioning, so long as the pop bumper is not so high as to not have at least a portion of the mechanism at the same level as the lowest flippers.

Be able to reasonably return the ball into the field of play (posts, lane guides, and similar obstructions would disqualify a game from consideration).

As the name implies, must be an active bumper (passive bumpers do not count).

With the criteria in place, here are the currently identified middle-pop pins, in chronological order:
Palm Beach (Marchant, 1950)
Presently the earliest-known example of a middle-pop game, it is noteworthy that it was made by Marchant, a French manufacturer. Palm Beach features a four-flipper layout with the middle-pop positioned quite close to the lower flippers. Gobble holes are also in the same location, so there appears to be relatively high risk of losing the ball once the middle-pop is in play.
Marchant was a small player in the pinball scene. It is unclear if this design influenced the future middle-pop layouts or if they were developed independently. Regardless, it makes for a fascinating coincidence that the French arrived at the design slightly earlier than the American manufacturers.