Ok, interesting question here, I now have both a Dilog 703 and a Dilog 706. 
The 703 is a TMSCP tape controller and the 706 is a MSCP disk controller. 
(at work I have a 736 that does both, but I don't think they are the same)

The circuits are of course identical and I'm trying to figure out if Dilog 
used a PAL change like Viking did or just a firmware change between the two 
of them? If it is just firmware then perhaps I can help Ben turn his 
controller into a disk controller.

--Chuck