Mac – Can a flashed Mac Pro 2009 (5,1) support macOS Mojave

I think the model name shown in System Information is determined from information encoded within the serial number. It looks like the precise details of this are private to Apple, but some information is given in this MacRumors post (which includes the image below), and it looks like some sites may have reverse engineered it.

Be aware it is possible under some circumstances to change the serial number reported to the software running on the machine. Specifically: logic boards supplied by Apple to service providers have a "blank" serial number, and service providers must "serialize" them (this is only possible once, from the service provider point of view) by running some special software and providing the desired serial number via keyboard input. (This is why I suggested checking the serial number printed on the hardware in my comment on the question.)

Copied directly from Apple Discussions: Installing MacPro5,1 Processor Tray into MacPro4,1

[I can claim nothing but Google-Fu on this one. My SMC is the same as NoahL's 1.39f5 so I can only presume mine has the original 4,1 daughterboard, with 3.46GHz Westmeres added after-market. I bought mine ready-upgraded, so it's not a process I've ever done myself.]

From this, though, it would appear that the 2 versions of the CPU/RAM daughterboard are not interchangeable.

I think I can answer the question now. There was a misunderstanding regarding the firmware upgrade tool to upgrade a Mac Pro 4,1 to 5,1. The firmware upgarde tool does not update the SMC firmware, but installs a different boot rom version. Mac Pro 4,1 (2009): MP41.0081.B07 (original) Mac Pro 5,1 (2009): MP51.007F.B03 (upgraded)

The system and CPU board SMC version remains at 1.39f5 for the MacPro 2009. A Mac Pro 2010 uses 1.39f11. After upgrading a Mac Pro 4,1 to 5,1, the SMC firmware was still 1.39f5. SMC mainly deals with temperatures and fan speeds. It is important that the SMC version of the CPU and system are the same. So to answer my question, it is not possible to use a 5,1 CPU board in a 4,1 system or vice versa. I'm not sure what happens when mixing them, but apparently the system does either not boot or the fans go crazy, perhaps depending whether you put a 5,1 cpu board into a 4,1 system or vice versa. Here are some links I found meanwhile for reference:
http://forum.netkas.org/index.php?topic=852.0;wap2 http://forum.netkas.org/index.php/topic,852.375.html http://arstechnica.com/apple/2011/05/firmware-hack-can-transform-a-2009-mac-pro-into-a-12-core-monster/

Btw, the firmware upgrade tool 4,1 to 5,1 shows error 5570. That is because Apple changed the location of the 5,1 efi firmware image. You can however download the 5,1 firmware from Apple and simply mount the disk image prior to running the tool and it will work fine. Then shutdown the system and hold down the power button until a tone occurs, then the system will restart and show a progress bar to update the firmware.