Different hardware may provide different specifications, news at 11.
You either have to ask TP-Link for the details (which they aren't going to answer) or study the FCC tests and take a microscope to the PCB, to reverse engineer the rf path and the components used therein.
All are v4. I have no v3. Only difference is one v4 bought about one year ago, and the other two bought the last days. That is why I am so confused.
Edited the first post, it was typo. All are v4 devices.
I've ordered new WR802N from other seller and these have 25 dBm maximum transmit power.
SN beginning with 219 and 220 maximum transmit power 25 dBm
SN beginning with 221 only 22 dBm maximum transmit power (ordered from Amazon).
But also: Both 219 serial number and 220 batches are from "TP-Link Italia", whereas 221 batch is from TP-Link Germany. So maximum transmit power might not be dependent on serial number, but by production department.
If maximum transmit power is dependant on serial number: From my experience Amazon has quite a small stock so you usally get the latest version of products. In this case this is not optimal. Try to get if from a medium sized seller. Thats where I got the batches with SN220 and 25 dBm transmit power.