I've recently tried to do a cold flash after normal flashing doesn't appear to fix constant crashes of my phone. The instructions I've found all refer to hardware revisions in the 2xxx range - however the instructions say that the hardware revision id is on the label. Most of the text is rubbed off the label on my phone, but i can make out the part before RX-51 as being 661E (identical to the example label on the firmware download page where you enter the IMEI number).

Supported revisions for firmware are listed, but I've not seen any 6xxx numbers listed.
The phone reports as being 2204 when flashing, but I'm wondering if the previous owner overwrote the settings by cold flashing himself.
Should I attempt to cold flash using 661E as the HW id parameter?

I've made sure to flash in the correct sequence, too - emmc first and then the other.
I may try dual booting into android to see if that's more stable - that may help me to diagnose as to whether it's a hardware or software issue.

There's two other ways to see your hardware revision:

• Code:

  flasher-3.5 -i

  This should work regardless of which OS you use, as long as you use the flasher-3.5 through terminal/console/command prompt. You should see the output with the revision once the device is in firmware update mode.
• Code:

  flasher-3.5 --enable-rd-mode

  Apparently that only works if your host computer (the one that N900 is connected to) is not windows. In other words only linux and Mac versions of the flasher can enable r&d mode. Though once enabled you should be able to see green words at the NOKIA screen, the bottom line should be revision number:

Dualbooting with nitdroid (i.e. android for N900) will not solve the issue if you cannot flash it as nitdroid relies on multiboot that works with NOLO and cannot be easily installed separately when the device is not in the normal bootup mode with multiboot already installed.

I'm pretty sure the revision returned by -i is 2204, but wondered if specifying a revision when cold flashing actually overwrote the stored revision, or if it was to force installation of a different revision.
I don't quite understand why nokia says that the revision is on the back when it doesn't seem to be.

I do manage to flash successfully, it's just that the machine seems to be crashing frequently (even when in standby it crashed twice last night, waking me up).
Sometimes it'll run most of the day without crashing. I'm finding it hard to diagnose.

I personally would doubt cold flashing would be able to overwrite the hardware revision on the device. In fact even if presumably the revision was overwritten, information dumped by kernel when one does:
will not show the modified revision from cold flashing with a different revision. Though I am not willing to risk testing cold flashing on any of my device currently.

As for the revision number being written on the underside of the device. I had a quick check on my own device and could not find any relevent number matching my hardware revision number. I'm guessing the revision number were only on certain N900 that were never made for sale. They could be either pre-production units, prototypes or potentially engineer samples, etc. I found a photo here with the revision printed clearly on the sticker on wikipedia. Though I'm sure the same sticker found on that photo would not be the ones that we have now which is similar to the image from N900 firmware downloads page:

You can also see on the image above has the characters `661E' which were the ones you said you noticed, those are actually Canada ID not hardware revision

Thanks, I'll give the proc method a go just to double check then.
Edit: The command gives the revision as 2204 - so evidently using the right firmware version then.