Apparently you can unbrick using TFTP: https://www.trishtech.com/2015/06/unbrick-tplink-wr740n-wifi-router/
I have the same problem - the only firmware I can flash is a sysupgrade format of the one I already have installed. Otherwise I get the same error.
The reason I want to revert to stock is that the OpenWRT firmware installed doesn't keep settings through a reboot (uses a ramdisk for it's overlay) and I haven't been able to work out why.
Edit: I tried the TFTP recovery, and I can't get the device to go into this mode. I checked with Wireshark to see if there were TFTP packets, but saw none.