Nokia is expected to push into China very soon e.g. within weeks and now we're starting to see some of that effort. In a screeenshot of the the Lumia 800c ('c' is presumably for China's CDMA system) we can see that this is now rocking a later version of "Tango" than build 8757 (which we've seen lately on some other phones e.g. TITAN II, Lumia 900) but it is the same OS as found on the Lumia 610, lending credence to the idea that 8763 may be close to the shipping ROM. (See below for updated info).
In our discussions with Nokia, they noted that there is no final-build of "Tango" though and in fact they don't refer to it internally as that. Evidently, they just get new builds which they use to address issues accordingly, which makes sense. Understanding that it makes sense that "Tango" is not a fixed build but rather a range of post-Mango updates and with certain ones reaching "shipping" status.
Also of interest is the later firmware build. Here the Lumia 800c is sporting firmware 12080 which is just slightly ahead of the recently released 12070--once again it's safe to say this has more to do with the radios on board than anything significant. There's little doubt Nokia has to customize things a bit for China's more strict ecosystem so these changes are to be expected.
Update: Now we have a second image with OS build 8765 and firmware 12090. Worthy of mention though is "Main_ship" implying that yes, this is the final ROM for China.
Source: WPXAP; Thanks, talan1314, for the heads up!