That's a good remark, Alberto. To elaborate: a triangular or square wave with fundamental frequency f0 = 1.45 Hz (and not 2.9 Hz) would produce spectral peaks at odd multiples of that frequency (2.9 Hz spacing), i.e. at 1.45, 4.35, 7.25, ... , 21.75, 24.65, 27.55, 30.45, 33.35, etc. Hz. These are shifted by 1.45 Hz from the frequencies reported in the first message. On the other end, a sawtooth wave with f0 = 2.9 Hz would generate harmonics at 2.9, 5.8, 8.7, ... ,  20.3, 23.2, 26.1, 29. , 31.9, 34.8, 37.7, and so on.

This means that it is more likely that the signal originates from a sawtooth wave with fundamental frequency f0 = 2.9 Hz, filtered to give the appropriate amplitude scaling as described in the previous entry.