Sorry to hijack the thread back ONTO topic, but . . .
Maybe fellow ex-Chicagoan Bluezebra remembers this also, but I used to umpire 16-inch under UPA (Umpires' Protective Association) rules. The only difference I really remember is that if the ball fell into the little "triangles" at the back of the plate, it could not be a strike (same as hitting the plate).