The only difference between a 690 and a 570 is the badge (number) and the ROM. Everything else is the same. Most of the 690s I have seen even have the same ROM (software wise), but it is an EEPROM rather than a ROM as the 570 has. This makes it hard to call the 690 a true prototype. The only reason Commodore changed the name (number) was because of the new computer about to be released, the A600. They thought there would cause confusion with the 690 designation.
At least this is how this one ws filtered down to me, from a usually reliable source at Commodore.
