By the way guys, I just noticed there were two slightly different versions of the Arp Quartet. I wasn't aware of this until now when I went through all the pics I have of this model.
The first one looks like this (presumably the original version):
This appears to be the most common version of the Quartet, it's the one pictured in the original Arp adverts from 1980.
The back panel has the following text silk-screened on it:
The other version of the Quartet looks like this (this is the one seen in the video above):
Unlike the version pictured above, the back panel of this version does not say "Manufactured by Siel S.p.a. for Arp instruments". Hmm..I wonder
"The (Yamaha) CS-80 is a step ahead in keyboard control, and a generation behind in digital control" -- Dan Wyman, Jan 1979