Thanks for all the responses, I'm now pretty sure I have one I can just upgrade. I'm going to try the upgrade because there's a couple of things I like in the firmware that my device doesn't do.
That and I hope it'll fix an issue I've been having. My laptop died on me whilst my onyx m92 was connected to put some books on there. So it didn't get disconnected the way it should have been. Since then, every time I turn on my ebook, it first opens a page of a book that's no longer on there. Then I have to wait until it closes said page. After that I have to wait until my reader opens the book I was actually reading. Quite annoying. Yes I have done a full reset of the device and put all my books back on after that, but the issue persists. So I hope the other FW will fix it.