The "Trap-door" is my own play of words off the "Tra-pocket" and "Trap-eze". This model was invented by Edward Redmond in 1887. When I first saw the patent drawing, I thought this can't be right, it is a funnel emptying into a pail of water. Actually, the drawing shows a leather "Hand Protector for Ball Players". The top drawing shows a side view of a leather cone with two riveted leather "handles" to insert the wrist and hand. The bottom drawing shows a front view cross section with the little elastic trap door(B) and the tension spring(C) below. This device was to allow a ball to pass into the cone but not bounce back out because the tension spring closes the trap door. The patent document recommends lining the inside of the leather cone with india rubber and padding at the bottom to absorb impact of the thrown ball
I made a mock-up to see what the mitt would look like.