If California
Digital is still in business, I believe the
 boxed CP/M they were (are?) selling is for the Xerox 820... 
 That is certainly possible, Doug.  However, the one that they were
 selling a while back (for $9.95, IIRC) was for the Xerox 1800 terminal
 less "laptop".  Whether it is directly compatible with the 810, I do not
 know. 
 
This purchase was circa 1985...
But that's the right price... I'll just have to look at
the disks when I get home again...
-dq