Sigma Electronics Systems Display Generator 5564

Can anyone tell me anything about the subject: line? It seems to be a high-end graphics terminal from the mid 1970s. I have found the electronics unit while unpacking here, I think I have the keyboard (with a little built-in joystick?) somewhere too. I don't have the monitor, but I am guessing it's TV rates. Pysically it's just a metal box (about the size of a 4U rack unit without flanges). There is nothing on the front panel. The back panel has the on/off switch and connectors for a keyboard (DA15), 'Digital Video' (ditto), Host and Downstream RS232 ports (DB25, of course) Host and Dowstream current loop ports (DE9), 3 video outputs (BNC, I assume these can be used as R,G,B), mix video output (ditto) and video input (my guess is that the terminal video can be overlayed on some other video signal, again a BNC). The top cover comes off with 2 screws. It reveals very little apart from a massive linear PSU. The interesting stuff is exposed by removing the front panel (4 screws) which gives access to the card cage. There are 2 columns of half-width cards at the top, then about 7 full width ones. 2 of the half-width cards are wire-wrapped not PCBs. I forget what they do, one of them seems to be the video timing/sync counters. On other half-width cards are the processor (6800), ROMs (lots of EPROMs),Comms, video output (resistor DACs, etc) and a 'vector generator' which I guess is to draw lines, it's just a lot of of logic ICs, no microprocessor. One of the full-width cards is something like 'area fill'. So it does that in hardware too? The others -- and there are half a dozen of them -- are 'Pixel Store'. Each has 16K*16 of DRAM and a 16 bit ALU (4 off 74181). I hate to think how much this cost when new. RAM and ALUs were not cheap! -tony