Here is very bad scan from Festool's 2001/2002 global catalog.
Basis 5 is pre-CMS module for OF-1010, Basis 6 is module for OF-2000. I think it should work with modern CMS system too.
This looks to be functionally equivalent to a pin router attachment. We also make one - and have instructions posted at the link below which you may find helpful!
It is indeed from the Basis and allows you to follow a top mounted template. I don't believe it is a seamless fit on the new CMS but it should be adaptable with a little work, probably adaptable to a more 'pedestrian' table as well. I know a fellow that has one and he sang its praises as one of his most useful router table attachments.
I guess it will sit in its box for quite some time then, since I don't see any use for it. Being able to copy most things with a copy-bit or the guide ring.