RARE 1948 menstrual cycle forecasting paper slide rule. Once the number of days in the cycle has been selected on the slide #1, and the first day of flow indicated, then the slide rule calculates the first day of flow in the next cycle and highlights the days when pregnancy can occur. Great conversation piece or ideal gift for your OB/GYN!! Excellent condition and includes the original mailing envelope which is postmarked New York, New York on March 29, 1948. Can't be too many of these out there!