According to
@huitong 's CMA-Blog, "The latest video (January 2021) indicated that a few Mi-171Es started to wear a new desert camouflage for the Gobi desert in Northwest China (S/N 997xx)" like this one with the serial number LH99728.
However I'm confused since the new PLA AA serials are 6-digit numbers and LH997xx is rather unusual.
(Image courtesy of CMDSY via Huitong's CMA-Blog)
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I know this is a whole month later, but I just saw this now.
You mentioned the serial number is unusual, I noticed another small detail, not sure if there is any meaning to it.
The "Mi-171E" stenciled in the panel is in Russian/Cyrillic. (Ми-171Е)
If this is a new paintjob, it means they actually painted the Russian back on?
Seems like an unusual thing to do. If we take Il-76MD as an example. Russian ones naturally have “ИЛ-76МД”. PLAAF models don’t have the model marking from pictures I’ve seen. Most other export models (India, Iran, etc.) have “IL-76MD”.
Doesn't answer your question whatsoever, but maybe it means something to you.