The 1961 FPI-15B and 1963 FPI-16B have the same exterior dimensions. The 1963 models got rid of the cold plate in the refrigerator section. The interior volume of the refrigerator section is marginally bigger because of that but it's simplified design means one less fan motor, one less thermostat (the one for the fan in the refrigerator section of the 1962 and earlier models is hidden and it's adjustment is a service operation) and no cold plate in the refrigerator section. For all the parts removed, there's one extra part and it rarely fails in the 1963, that part is a thermostatic valve that regulates the airflow in the refrigerator section, it works in a similar way as a coolant thermostat in a car.
The 1963 models are still more complicated than manual defrost refrigerators but the simplified refrigerating system works better at maintaining constant temperatures in the refrigerator section when it works fine.
One thing to look for on all these early Frost-Proof Frigidaire refrigerators is to see if there's a current leak on the cabinet. If the refrigerator has been stopped for a while, it might not be detectable but if it's running, you can disconnect it and test put one of the probes on both prongs of the power cord and the other on a metallic part of the fridge like the stainless trim around the cabinet. With the multimeter set on "ohm" or "beep" (if it has the beep feature), it should not beep or detect continuity on that setting. Another way to test that is to ground one of the prongs of the multimeter and touch the cabinet of the fridge while it's connected with the mulltimeter set on 200V AC, see if it shows current leak, repeat the operation with the fridge's plug reversed in the wall outlet (these aren't polarized). If there's no current or minimal current leak, it will show 0 or a very low reading, if it shows something like 118 volts, it will likely need a new defrost heater. The defrost heaters aren't that easy to replace on these refrigerators.
Also, before you fill it with food, make sure that the temperatures in the refrigerator and in the freezer are OK and that the defrost timer and heater work correctly, every 12 hours on a 1963. The timer's manual advance shaft has been moved from just behind the kick plate on the left side to facing the water tray also on the left side. That is of course if the timer hasn't been replaced with a generic one.
To test the temperatures in the freezer and in the fridge, I suggest placing a good quantity of water in a large plastic pan in the freezer and a good quantity of water in a pitcher in the fridge. Set the cold control to "1" and wait for a day to see what happens. Then put a thermometer directly on the ice in the freezer and in the water in the fridge. You want the compressor cycle on and off occasionally (not just twice a day when the defrost takes place but a few times between each defrost periods and check the temperatures of the water and the ice in the freezer. The ice should be at or below 0°F and the refrigerator section should be between 32°F and 40°F.
The only disadvantages I see versus the earlier cold plate system is that the air that circulates in the food section also circulates in the freezer section. So if you keep things for a long time in the freezer, things may catch odors from the items stored in the refrigerator. But most of today's fridges are also made like that and they circulate the air between the freezer and the refrigerator sections.
I have one 1963 FPI-16B and it works nicely.
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