header #24 is indeed one of the two optional pwm fan headers (the other is #23) and the fan header has a tab protruding upwards that runs across all pins, instead of just 3 as found on most other 4 pin motherboard pwm headers
if your fan connector has a center ridge you will either have to mostly remove it at the end of the connector to allow you to push the connector on the 4 pins or learn how to slip the header onto the motherboard connector by pushing it on the pins using the proper starting angle