On Ebay Chinese boards for the Quad 405 are for sale.
We ordered some to try them out.
They are based on the first version of the 405 board with some modifications in the in- and output connectors.
They will not fit the original aluminium heatsink-profile and they are 10mm longer and 5mm larger than the original board so they won't fit in a 405 but they may be interesting for DIY-projects in another chassis.
They come without schematic diagram or documentation.