I've just seen this on eBay
https://www.ebay.co.uk/itm/Vintage-Sony-SQD-2020-SQ-Decoder-Quadraphonic-Working-Order/112715155167?hash=item1a3e58c6df:g:~GkAAOSwhqhaQ92J
Very good BIN price, so if you fancy having a go at Quadraphonics, this is well worth while. I have one, and it's pretty damn...