Yes, it almost certainly is a heat exchanger as John said. Engine coolant would flow through one set of connections without going anywhere else. The other pair of connections would be plumbed to the fire pump with heater size hoses to cool the engine. It is useless on a truck without a pump and generally a trouble spot for leaks.

If you do not have a fire pump, get rid of it. If you do have a fire pump, get rid of it. It will be a constant source of leaks and of no operational value unless you pump for a long time at a big fire. I am gonna guess that is unlikely.

Mac :{)

1962 K10 short step side, much modified for rally
1969 T50 fire truck, almost nos, needs a few things