I expect he took a 3.1 ROM image patched with the 3.5 updates and burned it with an Eprom burner. There's a 3.9 ROM image too that some Polish guy burned to a ROM and put in his A1000. On an A500 or A600, a A 3.0 ROM will allow AmigaOS 3.1, and a 3.1 ROM will allow 3.1 + 3.5 + 3.9.
