The manufacturer produced the backglasses in both crystal glass and Plexiglas.
In the middle area of the motor board is a 10-pin jack with a 4-pin shorting plug. This shorting plug is required for replay operation. This jack will accommodate an Add-a-ball relay, likely ordered separately by the operator, required for Add-a-ball operation. The operator would remove (and save) the shorting plug and attach the Add-a-ball relay to the nearby vacant area of the motor board, plugging it into the vacated jack. The schematic refers to the "add-a-ball relay".
