odd console cable

[barmar@Think.COM (Barry Margolin)]

    Date: Mon, 30 Mar 1992 12:54 EST
    From: Scott_Busse%mindlink.bc.ca@Warbucks.AI.SRI.COM (Scott Busse)

    I purchased a monochrome monitor from ADS, which
    turns out to be for a 3600, apparently not at
    all compatible with a 3650. 

Right.  The 3600 console is not compatible with any other 36xx models.
It used a different video encoding; the others use Phase-Encoded video.
There was an upgrade to 3600's to allow them to use PE video, but I've
never heard of a mod to the later machines to allow them to use a 3600
console.

				Along with it was
    the console cable, which had the same connector
    on one end as the '50, but on the other end
    it was hacked into a male BNC connector, and
    a db15 female. Does anyone know what this is
    for? My assumption would be that the BNC part
    is the video signal, and the db15 is somehow
    the keyboard/mouse signals...except that I
    no longer make assumptions :) (right Dave?)

This sounds like the "wall-mount" cables.  For going long distances
through wall conduits, the signal was split onto two cables, as you
surmised.  At each end there would be a wall-plate that recombined them
and connected to a short console cable (although in your case it sounds
like someone just split the cable itself instead of using the
wallplate).  In the machine room the wall-plates would probably be
mounted into a patch panel.

                                                barmar