OFDM-30 (+1 pilot, +2) MSK 50Bd possibly Chinese modem?

The signal was recorded at 8403.5 KHz (CF) thanks to the use of some KiwiSDRs [1], it occupies a bandwidth of 2500 Hz and seemigly consists of 35 tones (Fig. 1a). The lower tone (in case of USB) most likely is the "pilot" one, used for Doppler correction, and its level is 7 dB higher than the normal level of any one of the the remaining 34 tones (Fig. 1b). The pilot seems followed by four tones: actually two PSK2 channels modulated at 25 and 50 Bd.

The remainig 30 tones are used for data transfer, they are ~71 Hz spaced and are formed using the OFDM technology. Curiously, the transition from idle/data phases does not happen simultaneously for all the channels, the delay is approximately 3500ms starting from the lowest channel (Fig. 1c).
A similar multi-channel signal was already meet here (thanks pir3 for his comment in twitter).

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Fig. 1a

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Fig. 1b

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Fig. 1c

The two lower 25Bd and 50Bd PSK2 channels after the pilot tone send a continuous sequence of zeros and ones which is most likely used for sync purposes.

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Fig. 2a

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Fig. 2b

The analysis of the 30 data tones shows a 4-ary constellation in absolute mode and a 2-ary constellation in relative mode (Fig. 3), therefore in my opinion these tones are keyed using MSK modulation with symbol-rate of 50 Baud and 25Hz shift; the analysis of a single tone confirms my guess (Fig. 4). No particular patterns were detected during the data phase.