I've got a Kenwood Chef 901 which I was given for a wedding present way back in 1967 and it's still in frequent use.
According to my manual the red knob is called the "slow speed attachment locking catch" and when you move it to the right it releases the "slow speed outlet cover".
You can get various attachments that go into this outlet and they are listed in my book as a coffee grinder, mincer, slicer & shredder, cream maker, bean slicer and can opener.
You can still buy the various attachments on eBay if you fancy anything. There is also a cover on the top at the back of the mixer which flips off to give access to the "high speed outlet cover". Onto this you could attach a juice extractor, liquidiser, high speed slicer & shredder and potato peeler.
The instruction manual shows all the various attachments but is more than half full of recipes as well. If you need any of the instructions or diagrams just let me know and I can scan and email them to you