Ok, the device I received was apparently on old firmware, but I made the mistake of working on it while plugged into the internet, and it upgraded itself (probably overnight)...
w6$ cat p9/event_monitor/event.log | egrep -a -B7 -A2 'FW Upgrade'
[2023-02-24 07:05:13][get_versionType:23488] -------- OS Version: W6_1.30.308516 --------
[2024-08-28 02:13:28][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: DM Client
[2024-08-28 02:13:28][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: LED Client
[2024-08-28 02:13:28][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: App Adaptor
[2024-08-28 02:13:54][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: WiFi Manager
[2024-08-28 02:13:55][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: Network Server
[2024-08-28 02:14:04][process_CMD_UM_SET_CLIENT_READY:2817]Client Ready: Web Server
[2024-08-28 02:14:43][system_reboot_action:23300]ResetEvent: FW Upgrade
[2024-08-28 02:14:57][main:23676]Start UM(0) : YX5.20230823.720f28a
[2024-08-28 02:15:12][get_versionType:23491] -------- OS Version: W6_2.15.402115 --------
[2024-08-28 02:17:07][process_CMD_UM_SET_CLIENT_READY:2818]Client Ready: DM Client
[2024-08-28 02:17:07][process_CMD_UM_SET_CLIENT_READY:2818]Client Ready: App Adaptor
[2024-08-28 02:17:07][process_CMD_UM_SET_CLIENT_READY:2818]Client Ready: LED Client
[2024-08-28 02:17:07][set_update_result:1301]GET_UPDATE_RES: Status=1, Command=1, OS_VER=W6_2.15.402115, ErrMsg=None
[2024-08-28 02:17:07][process_CMD_UM_GET_UPDATE_STATUS:2212]FW Upgrade: success=1 version=W6_2.15.402115
[2024-08-28 02:18:27][process_CMD_UM_SET_CLIENT_READY:2818]Client Ready: WiFi Manager
[2024-08-28 02:18:28][process_CMD_UM_SET_CLIENT_READY:2818]Client Ready: Network Server
Furthermore the new image included a new BL2/fip.bin
w6$ cat p8/etc/vendor/cust_swver
W6_1.30.308516
w6$ cat p6/etc/vendor/cust_swver; find | egrep -i 'bl2|fip'
W6_2.15.402115
./p6/usr/bin/fip.bin
./p6/usr/bin/bl2.img
and this matches what is there now:
w6$ cmp -l mmcblk0boot0 p6/usr/bin/bl2.img
cmp: EOF on p6/usr/bin/bl2.img after byte 241720
So basically I bought a new but -presumably- working due to old fw router, and it upgraded itself before I managed to get serial/openwrt on it.
Anyone with a still automatically booting OpenWrt care to share a dump of their partitions with me? Maybe I can still flash back assuming some anti rollback hasn't been triggered...