This is one Johnson reel model that I haven't had in recent years that I recall our family having at least one of when I was a kid. The Johnson 120 Centennial spin-cast reel reel came out around 1957 I think. It was made quite differently than other Johnson spin-cast reel models. It was a solid reel. The back part that contained the gears was made of cast metal and was sealed similar to sealed bearings. They were not intended to be serviced at home as were most Johnson models. The drag of the Centennial worked better than did the concurrent Century 100 and Citation 11o models. The main apparatus of the drag appears to have been very similar to the later model drags used in the beefier models such as the SANGA, and the 710.
The spools were easily changed for different line weights or types with the Centennial, just as were the Century, and I believe them to be the same spools.