Some more datapoints. I have a 841N(TPD) ver 9.0, ver 10.0 and ver 11.1, all bought in EU.
For the project I'm working on I need 15.05.1 but I had to re-build the firmware image to eliminate things I don't need, to get space for the things I want which would not fit otherwise.
Chaos Calmer 15.05.1 r49389 / LuCI for-15.05 branch (git-16.344.58491-d4bec26)
I note that the 9.0, the 10.0 and the 11.1 hardware flashes correctly, from the stock TP-link firmware, using the image I built earlier today:
where XX is v9, v10 or v11 respectively. No need to patch the images; just configure, build, flash and off it goes. That may not be true for the 15.05.1 release images, it is true when using the images I built today.
There are two quite visible hardware differences between v10.0 and v11.1 hardware.
For one, the WAN LED is now bi-color, can be green or orange. Flipping the tp-link:green:wlan LED makes the LED light up orange, not green, green still seems to indicate WAN activity.
The second difference is that the shape of the antenna is different, the v9.0 and v10.0 box have identical antennas; the v11.1 antennas are bigger and look like the antennas used on Archer-C7 machines. They are still non-detachable though.
One more comment: these units have 4MB of flash and that is "taxing it" with 15.05 firmware. I use these units for a multi-unit wifi network (at a unit price of EUR 17 they are hard to beat) but I saw someone wanting to experiment creating an eduroam proxy and the like. Please do realize that the flash constraint does make for a challenge, if this is your first OpenWRT experiment then I would suggest to use a box with more resources (like the aforementioned Archer C7) because you can use the stock OpenWRT image and just add packages, which you cannot do with the '841 because you will run out of space.
Hope this helps,