Take a look at these.
http://www.ebay.com/itm/Edwards-1526Fire-Alarm-System-control-Modulesmokepull-station-AS-IS-/261652242186
They’re made by BRK (probably rebranded as Edwards) and notice #1 how they use the 77R/76RS’ sensor and #2 the PCB has 800 Series printed on it. I saw these at the Mark Twain museum years ago and thought they were some variant of the 769AC. I think these look like the offspring of the 769AC and an 1839 series.
BTW I asked the seller and they said if the whole system doesn’t sell they’ll part it out.