Squirrel tail ovens have an interesting flue which curves back over the top of the oven dome. "Beehive ovens" are small enough to have the flue exit hole just inside the door. However, if the brick oven is long, such as 7 feet, having the vent hole in the rear of the oven increases the air flow. After crossing the length of the oven, the flue vents into the chimney. The three photographs show 1) a square exit hole in the center back, 2) flue going up from the back of the oven, and 3) length of the flue above the oven. The links below have either good pictures or use the oven, most have classes.
PA Abbott Inn. Bucks County
PA Dill's Tavern. Dillsburg
PA Horn Farm Center. York
PA Landis Valley Village & Farm Museum. Lancaster
PA Mercer Museum Folk Fest. Doylestown
PA Peter Wentz Farmstead. Worcester