The pattern of those two sets of holes do suggest that two different style hot water heaters were once installed in your car. The one set of holes must have been done by someone in a big hurry to get a heater in that car
....maybe his girlfriend had refused to ride with him untill he had some heat
.
Small hot water heaters like you want seem to come up often at shows/swaps. I picked up one that was new, still in the box, last year and another included in some parts recently purchased. I am thinking about installing these in two of my cars with original uncut/unholed firewalls, by running the hoses under the passenger side of the firewall and up threw an aftermarket wooden floor board.