Onyx 5.6
Yes there was a 5.6 - actually it was the first time the rip took on its current user interface and thus the earliest one that current users could comfortably go back and use.
Thats where the good news ends though...
There are no downloads anymore and even if you found someone who had the disks, the upgrades etc were done online and that service has gone so you wont get the bug fixes / service packs etc.
You also cannot upgrade this rip any more - not since 2010.
Also as the rip was launched in 2000 or 2001 then any new printer beyond probably 2003 will not work with it.
Thus you need to find a printer launched over a decade ago to work with it.
I hope the person that sold you this dongle mentioned all of this before sale?...