74LS 14 (the questioner is wrong! ) is a six Schmidt inverter. Two Schmidt inverters in series are used for pulse shaping, that is, when the input pulse waveform is uneven, there are spikes and burrs, the pulse is turned into a square standard pulse to prevent false triggering caused by uneven pulse waveform. As for why there is no pulse shaping circuit in the second picture, it is unknown and may be omitted.