I bought it on eBay: https://www.ebay.co.uk/itm/175882294924

It uses a 68B09P (6809 rated at 2 MHz, with on-chip oscillator), and a 7.7328 MHz crystal.

The UART is a 68B50P (6850 rated at 2 MHz), which has problems with USB-to-serial adaptors made by FTDI due to a bug in the latter's chips.

See https://hackaday.io/project/167418-ftdi-usb-cable-problems-with-6850-acia

It is nothing amazing, just a layout of Grant Searle's minimalist design. 

It does have a 40-pin expansion socket, which makes life easier if you wish to add experimental features (which I do).

In particular, I'd like to add a better serial channel device, to replace the 6850.