I have owned a Suburban with FFS and Positron II with the same decals as the OP's bike. I know the two systems were typically paired together but I was under the impression that they could be used separately, the only difference is it would not be indexed shifting. I agree though, the large cog on the freewheel looks strange and the rear derailleur seems out of place. One other thing to note, my Suburban had a much different style of chain ring so maybe this bike was fitted later on. You can see it still has the Schwinn twin stick shift levers so it's definitely friction.

Here is my old FFS/ Positron II Suburban to show the differences (yes the seat clamp is on backwards)