I've got an ancient Teledyne Laars Mark V heater. The pool and equipment were here when we bought the house 18 year ago and I wouldn't be surprised if this heater was installed in 1978 when the original owner built the pool.
We never use the heater. We live in Southern California and have solar so we don't really need it. The outside of the heater has gotten so faded that I can't read the label on the on/off switch. Is the heater on when the switch is up or down? I probably should just turn off the gas to the equipment, but I might want to actually turn it on and see if it is working someday. I'm currently preoccupied with installing a new variable speed pool pump and then I've got other deferred maintenance, so it probably won't be any time soon.