The response I've received so far from communicating directly w/Reolink Support is that the IPC_51316M is indeed a new version of the 4MB RLC-420, and hence has no new firmware beyond the version they are currently shipping with.
I've asked them to provide just a touch of further clarification on that prior to them closing the case, but I suppose what that means is that as soon as the first new, updated version of the firmware for the IPC_51316M becomes available, the download hyperlink for the RLC-420 will point to it? But if that happens, what would be the process for users visiting the site in search of firmware for their IPC_3816M based RLC-420's ????

I just received a new RLC-420 and was wanting to update it's firmware.
When I download RLC-420_1389_18081400 from /firmware/, my camera returns an err "Failed to recognize format'"
I do notice that when I am prompted by the UI to supply the path to the firmware file, it is expecting "*.IPC_51316M".
I then noticed that the PAK file in the firmware update looks like this: IPC_3816M.1389.....
From the Information page on my camera:
build 18060901, IPC_51316M, Config ver v2.0.0.0, FW ver v2.0.0.143_18060901, details IPC_51316M110000000100000, Client vers v1.0.23

Why is the available download for the RLC-420 not working for me?
ANY help or ideas on this would be greatly appreciated.
thanks