OK folks, the time has finally come for middle-pop games to get their own thread. Welcome!
Middle-pop pinball machines feature an uncommon but fun layout: A pop bumper between the flippers! This creates a wider than usual flipper stance and forces the player to learn new strategies of play. Trapping and passing the ball requires extra finesse to cross the gap and occasionally requires the use of the pop bumper. Many times the ball can be nudged back into play by using it alone, which often adds a fun level of unpredictability. Middle-pop games are thus a terrific way to spice up a collection of pins with traditional layouts and provide an opportunity to learn new skills.
I have combed the entire IPDB twice and, to my knowledge, the middle-pop + flippers layout began with Wayne Neyens' 1952 All-Star Basketball. A few months later in the year, Harry Williams designed Majorettes. Both have a similarly wide flipper stance and lower rebound slings:
"All-Star Basketball" http://www.ipdb.org/showpic.pl?id=54&picno=42764
The two designers each released a middle-pop game in 1953 as well... with Wayne's beautiful Marble Queen followed by Harry Williams with Lazy-Q. Notice how both designs again follow one another, now with twin center drains below the pop bumper:
"Marble Queen" http://www.ipdb.org/showpic.pl?id=1541&picno=1477
In 1954, Neyens released Hawaiian Beauty which was the only middle-pop that year and the only one ever produced with a cluster of three. BTW, from this point onward, take notice of the large area Neyens dedicates to the lower playfield area below the flippers. He repeats this "big lower area" style for his next (and last) two middle-pops:
"Hawaiian Beauty" http://www.ipdb.org/showpic.pl?id=1138&picno=19348
1956 sees a total of five middle-pops produced... and the last from the woodrail era. Two by Neyens (Derby Day, Score-Board) - again, notice his "large lower area" style below the flippers:
...and three by Williams (Cue Ball, Shamrock, and Tim-Buc-Tu):
Bonus round: Neyens designed a third middle-pop-ish game in 1956 called "Register". However, I don't think this one qualifies because the central bumper is a dead bumper (no action) and doesn't create a gap between the flippers... so it functions more like a bumper-shaped obstacle than a true middle-pop.