I knew the engineer Robert at Amiga Technologies Australia and he claimed to have patched the ROM with latest files in software and re-burnt the rom image to eproms so they didn't need to be loaded from software.
I don't know how true this is, but it may explain your situation here. As far as LEGAL un modifed kickstart roms go, 3.1 was the latest developed for 1200, 4000 etc.
As for non legal, I'd love a burnt pair of 3.5 eproms

)))
Pitty Amiga tech closed their doors too. No money in AMIGAs anymore, parts are getting harder and harder to find, and boards are only getting older.
I never got a chance to buy these 3.5 ROMs so I can't claim anything I have said is truth.