In actually reading that manual as I scanned it, I first noticed the mention of the operator's manual. Should have realized before that there would be one. Searched this morning.. and found this on ebay: 10281-90905 Operator's Guide Model 1640B/Opt 001 Programmable Serial Da This listing was ended by the seller on Fri, Nov 8 at 6:49 AM because the item is no longer available.
Ha ha ha rats. Maybe this thread prompted someone else to find and buy it. Oh well that's life. Now for a presumably very long ebay search string wait.