1x Y-20U, 2x Y-20B and Y-20U, both with WS-20
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On past satellite imager, I believe we've had confirmation that at least 4 Y-20B airframes (i.e.: powered by WS-20s) exist, of which two looked to be standard transports and two being "Y-20BU" tankers.
We know that the first Y-20B transport prototype had serial 7810. In this image we can see 7811 on another Y-20B transport (likely second prototype).
The Y-20BU in the image is 7813. So either 7812 or 7814 is the other Y-20BU aircraft we spotted on satellite I imagine.
The big question for me, is whether the "Y-20BU" -- or rather, whether all Y-20Bs -- are tanker/transports or not.
The Y-20U images we've had suggests that the centerline positioning of its drogue refuelling point, means that its rear ramp/door is not functional -- structurally I don't see how it can still be there. Which means it can't operate as a tanker/transport in the way that some Il-78 variants can, or how A400M can, because those aircraft retain their functioning rear ramp and their centreline drogue refuelling point isn't actually structurally integrated with the rear fuselage and ramp.
For Y-20Bs, if all Y-20Bs can be configured as tankers -- even if it is only by integrating wingtip refueling probe drogues and a modular/removable fuel cell that can be carried in the cargo hold -- that would be a significant added refueling capability as it means that more Y-20 airframes are capable of being tankers in general.
That wouldn't preclude a dedicated "Y-20BU" tanker (no transport role) being developed (which could be better optimized for the air refueling role than a standard Y-20B by removing the rear ramp and permanently modifying the internals of the aircraft to accommodate more fuel).
Such a "Y-20BU" would basically be the same as the current "Y-20AU" but powered by WS-20s (two wingtip pods and a centreline refueling point), while the standard "Y-20B" would all have the plumbing to accommodate two wingtip refueling pods and fuel cells in the cargo hold.
Anyway, I've written about this before, I'm sure there will be confirmation in due time if the PLA had the forward thinking to plan for this additional bit of modularity.
