It's a one-page ad, not a manual. I have seen some of these though, in simpler form, made by Humphrey, Hoffman and I think a Rex. Never a Ruud, which is the original automatic water heater. As water flows, it triggers the main gas valve to open. A standing pilot ignites the gas that heats the copper coil. There's an internal temperature regulator that reduces the gas flow when too hot. When the water flow stops, the main gas valve closes. Some areas had "coal gas" and the coils required regular cleaning, thus the doors.