Can you please give me a couple of examples of this obvious evidence of constant tweaking I seem to have missed completely?
Well, okay. Let me rephrase myself since i typed that right before i went to bed last night and might have been a little too enthusiastic.
The variations themselves are not obvious at all, but certain things definitely stick out to me from my background in poking around and reverse engineering embedded stuff like this... also I can't claim to know anything for certain about the Solo or how Arizer does things! But please consider my reasonings for thinking what I think. And for reference when I am comparing things, I am comparing them between a M102, a M107 and a M1E1.
1. Startup times. The M102 turns on if you've held the two buttons long enough for "idle" to blink once. The M107 will not start up unless you wait for "idle" to blink four times. The M1E1 will not start up unless you wait for it to blink twice. I would be willing to assume this is controlled in software. It's possible one of mine just acts different than it should.
2. Availability of the "true PA" function. I am not as good at reverse engineering hardware as software, but I couldn't find any evidence that the "true PA" mode isn't just controlled by the Solo's software. It can tell if no battery is connected and it will still work as a desktop unit, but it is also designed (at least the older ones) to use some battery power even with the PA present should the battery be present as well.
3. Heating curves. Remember when I was talking about moving a ceramic heater into a new Solo? I did that before I gave my M1E1 out to a friend. I don't have equipment to figure out if the temperature readings were accurate to what they should have been with the M1E1's stock bowl, but my biggest reason for doing that was to find out if the M1E1 with a ceramic bowl would attempt to heat it like it always does, or if the heating curve would adapt to properly heat the ceramic bowl. It got up to temperature pretty quickly like the newer ones do (at least compared to the very old models) but I think it was still running too cool as the units themselves are calibrated to different temperatures. However it did not completely adapt the heating cycle of the M102, which is much more of an exponential rise in temperature than the newer Solos do. Which brings me to my next point...
4. Temperature calibration! Of course i can't prove this with the equipment I have on hand but if different Solos are calibrated to different temperatures that counts for software changes
5. Battery drain. It's entirely possible this is due to the battery protection PCB and not the software, but I don't know enough about protection PCB design to say one way or another. My M102 and M107 will let you drain the battery and it will struggle to give you the very last of the power it can possibly squeeze out of the battery. Everything newer Solo i have owned (so M1B and higher up in letters) won't let you drain the battery that low. It will just refuse to heat and give you the "dead" light instead of heating anyway, but my point is the older Solos will allow the battery pack's voltage to go lower before shutting off.
And last on my awful terrible list for now...
6. PCB changes! Again i'm not the best at hardware hacking in the world but I have removed and inspected the M102, M107 and M1E1's PCB's side by side. Components have been moved and replaced, certain ones are done away with entirely. It's not surprising as Arizer isn't the type to buy 10000000 of every given part they need and make the same PCB forever, but if every solo PCB revision could be loaded with the same firmware and work properly, i would definitely be both surprised and impressed. However I still think if one were to have a firmware image that could be flashed onto the Solo, it would have to be specific to that PCB revision.
It's all speculation, of course. And that's not all, but I feel like those are the best points worth mentioning for now. I don't have the software figured out yet, but please feel free to PM me if you'd like to discuss breaking the Solo's firmware further
PS
@OF I remember you troubleshooting someone's solo and asking if the "recall last setting" feature worked properly. It only actually works about half the time on my M102 or M107, curious if you have any ideas? I have a feeling the old dying battery packs in them tend to make the solo do wonky things.