Steptoeandson, Yours is known as a straight bar curb bit. Far more severe than the Kimberwick. Yours is designed to be ridden with 2 sets of reins. The top rein is in the "ring " part and the curb rein has the choice of upper or lower slot. The bottom rein being called the curb rein and is usually half the width of the rein above. Still in use today.
Upon reflection as you mentioned 1930s, if it is very heavy iron, it could/maybe an old cavalry curb bit, as they used these…who knows? Horse bits are very hard to date unless they are nickle…usually denotes early period 1900s and prior to.