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 ????