Nice pictures.
Prototype 7812, which fills the gap among the existing 781X prototypes that we've had pictures of (7810, 7811, 7813).
Also, I think this picture pretty clearly shows there is no rear central fuselage refuelling drogue like on YY-20A.
I think the designation for this thing is probably best given as "Y-20B" where it is widely acknowledged that Y-20B at baseline is able to operate as a transport or a tanker dependent on whether it has air refuelling pods installed or not.
The "YY-20B" designation otoh may be best kept unattributed, as we cannot yet confirm if they may choose to develop a more dedicated tanker variant based off Y-20B (with more dedicated tanker capabilities possibly being enhanced permanent internal cargo hold fuel bladders, a central fuselage refuelling drogue like YY-20A, or possibly even a boom refuelling option on the central fuselage if the PLA chooses to go that route for certain future aircraft).