I am seeking hardware and software to add to my SONY RX551 computer
which will enable it to control (perhaps by IR) the SONY 400CD
juke-box immediately adjacent to it. I came across one system and to
my regret failed to save it.

Hi bdominus-ga,

the Slink-e controller is the easiest solution to your problem:
Slink-e Infrared Equipment Controller, Smarthome
http://www.smarthome.com/1331.html
If this is too pricey, you can sacrifice functionality for savings
with the RedRat Remote system:
"The RedRat2", RedRat
http://www.redrat.co.uk/
If you really want to save money and don't mind getting your hands
dirty, here are plans to build a controller yourself:
http://www.ee.washington.edu/conselec/A95/projects/mdw/fproj.html
Feel free to request any necessary clarification.

Good luck with future searches!

Hoping I was helpful,
not_you-ga