Did you check the Internet Archive or repositories like BitSavers?
yup, also emailed to several old dev mailing lists.
it seems that nobody cares anymore.
I received two constructive answers yesterday, anyway, with one of the user manual I need attached by email.
Not enough(1), but better than nothing.
(1) there are some quirks on chips, not well-documented by the official public documentation, well known on the reserved documentation under confidentiality agreement, and somehow known among devs, but we are talking about early 2000s and they didn't take any notes, and don't want to talk or just don't remember, so ... it's "lost information" at the moment.