I have an old Arvin recirculating heater in my 49 3100. It's orginal and is your basic inexpensive heater option for the 3100 back in the day. I have to manually open the heater valve just below the thermostat housing in cold weather for heat and manually close it during warm weather as I don't want hot coolant circulating through the heater when heat is not wanted.
You should be able to make out the connections in the pic.
49 216 recirc heater connection The smaller hose running along with the two heater hoses is the radiator overflow line.
