Factory heater delete

My 61 Plymouth Belvedere was a heater delete car and had no defrosters. I believe defrosters were federally mandated at some point in the 60s, maybe someone knows exactly when.
I was under the impression you could get (A bodies anyway) cars quite late with none.

"Up here" the parts store where my Dad worked, and later in his own parts store, and later another where I worked, all carried accessory defroster fans........All the school buses had them