Is this so? Because the FPA* were providing instruction on the safe period well before then, and I assume that this would have been of most interest to those who were not allowed to use more efficient methods (as well as no doubt a certain % who were just squicked out at 'putting things up inside themselves'). Only this morning dug out a 'safe period' calculator (1930s, German) to illustrate a talk I'm giving.

*Though I've no idea what the Catholic Marriage Advisory Centres of the same period were up to.