Another observation, that thick trace on the left (7th pin frm the bottom left, going up). According to the "standard" module (I guess thats what we'll call it, since its the same as the ones I have here, the F-3188) thats "Rx" which wouldn't be such a large trace, and certainly wouldn't go to multiple resistors. Is that soldered to the board? and if so, I'm guessing thats the Vbat pin, powered by the LM317 on board. That pretty much confirms what you said, this isn't the same pinout. But the fact that it seems the speaker and mic connections are in the same area, along with the button IO (bottom left), means it probably isn't radically different.
Figuring out the pinout won't be easy, but there are some clues. The IO voltage is based off the internal 1.8V reg, so when powered up, outputs will be either 0 or 1.8V, with enough current to drive an LED. Inputs can have weak pull downs that I believe are always enabled on the 8645.
Another clue might come from the pin behavior on power up. That is, hooking up the unknown pins on the left to a logic analyzer, and powering up the board.
I just tried to find that board on google. I could only find ones using the CSRA64215.. but the pinout on the modules on the board, appear to have the same pins soldered. Are these boards working? Because the CSRA64215 pinout is completely different, and looking at your board, makes sense.
Example:
https://www.thanksbuyer.com/bluetooth-receiver-board-module-lossless-bluetooth-5-0-receiver-qcc3008-b-type-w-external-antenna-kit-67653 Looks like the same pins are soldered on that as yours, but the module used has a completely different pinout:
https://www.aliexpress.com/item/32819647079.htmlBut I agree your module is definately note the same as the standard F3188. Perhaps they made a 8645 module, with the same pinout as the CSRA64215?